diff --git a/1306_expansion_statements/d1306r5.html b/1306_expansion_statements/d1306r5.html index 88e83879..ae084890 100644 --- a/1306_expansion_statements/d1306r5.html +++ b/1306_expansion_statements/d1306r5.html @@ -636,10 +636,10 @@

Contents

1 Revision History

This revision: Rewrote the prose and the wording.

-

[P1306R3] Expansion over a range -requires a constant expression. Added support for break and continue -control flow during evaluation.

-

[P1306R2] Adoption of template for +

[P1306R3] Expansion over a range requires +a constant expression. Added support for break and continue control flow +during evaluation.

+

[P1306R2] Adoption of template for syntax. Added support for init-statement, folded pack expansion into new expansion-init-list mechanism. Updated reflection code to match P2996. Minor updates to wording: updated handling of switch statements, work @@ -647,15 +647,14 @@

[P1306R1] Adopted a unified syntax for +

[P1306R1] Adopted a unified syntax for different forms of expansion statements. Further refinement of semantics to ensure expansion can be supported for all traversable sequences, including ranges of input iterators. Added discussion about break and continue within expansions.

-

[P1306R0] superceded and extended [P0589R0] -(Tuple-based for loops) to work with more +

[P1306R0] superceded and extended [P0589R0] to work with more destructurable objects (e.g., classes, parameter packs). Added a separate constexpr-for variant that a) makes the loop variable a constant expression in each repeated expansion, and b) makes it possible @@ -672,9 +671,8 @@

for_each) or, potentially in the future, using the [:expand(...):] -construct built on top of [P2996R10] (Reflection for C++26) -reflection facilities, there are several reasons to prefer language -support:

+construct built on top of [P2996R10] reflection facilities, +there are several reasons to prefer language support:

First, repetition is a fundamental building block of algorithms, and should be expressible directly without complex template instantiation strategies.

@@ -687,7 +685,7 @@

return statement — and makes coroutines unusable.

-

Lastly, “iteration” over destructurable classes effectively requires +

Lastly, “iteration” over destructuring classes effectively requires language support to implement correctly.

Here are some basic usage examples:

@@ -770,9 +768,8 @@

For the last row, expand is -demonstrated in [P2996R10], define_static_array() -comes from [P3491R1] -(define_static_{string,object,array}) (although can be +demonstrated in [P2996R10], define_static_array() +comes from [P3491R1] (although can be implemented purely on top of p2996) and works around non-transient allocation (more on this later), and nsdms(type) is just shorthand for nonstatic_data_members_of(type, std::meta::access::unprivileged()) @@ -788,7 +785,7 @@

The expansion statement

-
template for (init-statementopt for-range-declaration : expansion-initializer) statement
+
template for (init-statementopt for-range-declaration : expansion-initializer) compound-statement

will determine an expansion size based on the @@ -802,19 +799,19 @@

{ for-range-declaration = E(0); - statement + compound-statement } { for-range-declaration = E(1); - statement + compound-statement } // ... repeated up to ... { for-range-declaration = E(expansion-size - 1); - statement + compound-statement } } @@ -937,7 +934,7 @@

expansion-init-list syntax -in [P1306R2].

+in [P1306R2].

In addition to avoiding ambiguity, it is also broadly more useful than simply expanding over a pack since it allows ad hoc expressions. For instance, can add prefixes, suffixes, or even multiple packs: {0, xs..., 1, ys..., 2} @@ -1037,7 +1034,7 @@

} -

Examples like this feature prominently in [P2996R10]. And at first glance, this +

Examples like this feature prominently in [P2996R10]. And at first glance, this seems fine. The compiler knows the length of the vector returned by members_of(^^T), and can expand the body for each element. However, the expansion in @@ -1055,9 +1052,9 @@

[P1306R3] revision, we instead propose -restricting expansion over iterable expressions to only cover those that -are constant expression.

+[P1306R3] revision, we instead propose +restricting expansion over expansion-iterable expressions to only cover +those that are constant expression.

In other words — the desugaring described above (which is similar to the desugaring for the C++11 range-based for @@ -1066,8 +1063,7 @@

members_of with the define_static_array function from -[P3491R1] -(define_static_{string,object,array}) we obtain a +[P3491R1] we obtain a constexpr span containing the same reflections from members_of:

@@ -1095,7 +1091,7 @@

expansion-initializer -(9.7 [dcl.struct.bind])

+(9.6 [dcl.struct.bind])

  • addition-expansion-declarations is:

    constexpropt auto&& [__v0, __v0, ..., __vexpansion_size-1] = expansion-initializer;
    • @@ -1270,12 +1266,12 @@

    [class.temporary]/5 -to clarify that there are now six contexts:

    +to stop counting contexts:

    5 There are five -six contexts in which temporaries are +several contexts in which temporaries are destroyed at a different point than the end of the full-expression. […]

    @@ -1286,23 +1282,26 @@

    7 -The fourth context is when a temporary object other than a function -parameter object is created in the -for-range-initializer of a -range-based -for -statement. […]

    +The fourth context is when a temporary object is created in the +for-range-initializer of +either a range-based for statement or an enumerating expansion statement. If such a +temporary object would otherwise be destroyed at the end of the +for-range-initializer +full-expression, the object +persists for the lifetime of the reference initialized by the +for-range-initializer.

    7+ -The fifth context is when a temporary object other than a function -parameter object is created in the +The fifth context is when a temporary object is created in the expansion-initializer of an -iterable or destructurable expansion statement, or in a full-expression +iterating or destructuring expansion statement, or in a full-expression in the expansion-init-list -of an enumerated expansion statement ([stmt.expand]). If such a +of an enumerating expansion statement ([stmt.expand]). If such a temporary object would otherwise be destroyed at the end of that +expansion-initializer or full-expression, the object persists for the lifetime of the reference -initialized by the expression in the expanded expansion -statement.

    +initialized by the +expansion-initializer, if +any.

    8 The fifth sixth context is when a temporary object is @@ -1341,7 +1340,7 @@

    structured-binding-declaration initializer + for-range-declaration: -+ $attribute-specifier-seqopt decl-specifier-seq declarator ++ attribute-specifier-seqopt decl-specifier-seq declarator + structured-binding-declaration + + for-range-initializer: @@ -1380,7 +1379,8 @@

    init-statement)., or
  • (2.5) for an expansion-statement, -its statement (but not an +its compound-statement (but +not an init-statement).

    • @@ -1401,7 +1401,7 @@

    S1 is a selection-statement, or iteration-statement, or -expansion-statement +expansion-statement, and S2 is the init-statement of S1, @@ -1434,7 +1434,7 @@

    4 -An identifier label shall not occur in an +An identifier label shall not be enclosed by an expansion-statement ([stmt.expand]).

    @@ -1494,7 +1494,7 @@ 

    expansion-statement:
-    template for ( init-statementopt for-range-declaration : expansion-initializer ) statement
+    template for ( init-statementopt for-range-declaration : expansion-initializer ) compound-statement
 
 expansion-initializer:
     expression
@@ -1503,21 +1503,22 @@ 
    expansion-init-list:
     { expression-listopt }

    2 -The statement of an -expansion-statement is a +The compound-statement of +an expansion-statement is a control-flow-limited statement ([stmt.label]).

    3 For the purpose of name lookup and instantiation, the for-range-declaration and -the statement of the -expansion-statement are +the compound-statement of +the expansion-statement are together considered a template definition.

    4 For an expression E, let the expressions begin-expr and end-expr be determined -as specified in [stmt.ranged]. An expression is iterable if it -does not have array type and either

    +as specified in [stmt.ranged]. An expression is +expansion-iterable if it does not have array type and +either

    • (4.1) begin-expr and @@ -1526,24 +1527,24 @@

      E.end() or

    • (4.2) -argument-dependent lookup for begin(E) -and end(E) -find at least one function or function template for each.
      • +argument-dependent lookups for begin(E) +and for end(E) +each find at least one function or function template.

    5 An expansion statement is

    • (5.1) -an enumerated expansion statement if its +an enumerating expansion statement if its expansion-initializer is of the form expansion-init-list;
    • (5.2) -an iterable expansion statement if its +otherwise, an iterating expansion statement if its expansion-initializer is an -iterable expression;
      • +expansion-iterable expression;
    • (5.3) -otherwise, it is a destructurable expansion statement.
      • +otherwise, a destructuring expansion statement.

    6 An expansion statement S is @@ -1552,13 +1553,11 @@

    for-range-declaration (including its implied initialization), together with the -statement; these -instantiations correspond to expressions resulting from the analysis of -the expansion-initializer -as follows:

    +compound-statement of +S, as follows:

    • (6.1) -If S is an enumerated +If S is an enumerating expansion statement, S is equivalent to:

      {
@@ -1567,21 +1566,22 @@ 
        …
   SN-1
 }
      -

      where +

      where N is the number of +elements in the +expression-list, Si is

      {
   for-range-declaration = Ei ;
-  statement
+  compound-statement
 }

      and Ei is -the ith -expression in the +the ith element of the expression-list.

    • (6.2) Otherwise, if S is an -iterable expansion statement, +iterating expansion statement, S is equivalent to:

      {
   init-statement
@@ -1589,58 +1589,59 @@ 
        static constexpr auto begin = begin-expr; // see [stmt.ranged]
   static constexpr auto end = end-expr;     // see [stmt.ranged]
 
-  static constexpr std::ptrdiff_t N = [] consteval {
-    std::ptrdiff_t result = 0;
-    for (auto i = begin; i != end; ++i, ++result) ;
-    return result;
-  }();
-
-  S0
-  …
-  SN-1
-}
      -

      where + S0 + … + SN-1 +} +

      where N is the result of +evaluating the expression

      +
      [] consteval {
+  std::ptrdiff_t result = 0;
+  for (auto i = begin; i != end; ++i, ++result) ;
+  return result;  // distance from begin to end
+}()
      +

      and Si is

      -
      {
-  static constexpr auto iteri = begin + i;
-  for-range-declaration = *iteri;
-  statement
-}
      -

      for all i in the range [0, N). -The variables range, +

      {
+  static constexpr auto iter = begin + i;
+  for-range-declaration = *iter;
+  compound-statement
+}
      +

      The variables range, begin, -end, -N, and -iteri are -defined for exposition only.

      +end, and +iter are defined for +exposition only.

      [ Note 1: The instantiation is ill-formed if range is not a constant expression ([expr.const]) — end note ]

    • (6.3) -Otherwise, let N be the -structured binding size of the type of the -expansion-initializer; +Otherwise, S is a +destructuring expansion statement and S is equivalent to:

      -
      {
-  init-statement
-  constexpropt auto&& [u0, u1, …, uN-1] = expansion-initializer ;
-  S0
-  …
-  SN-1
-}
      -

      where +

      {
+  init-statement
+  constexpropt auto&& [u0, u1, …, uN-1] = expansion-initializer ;
+  S0
+  …
+  SN-1
+}
      +

      where N is the +structured binding size of the type of the +expansion-initializer and Si is

      -
      {
-  for-range-declaration = ui ;
-  statement
-}
      -

      The +

      {
+  for-range-declaration = ui ;
+  compound-statement
+}
      +

      The keyword constexpr is present in the declaration of -seq if and only if +u0, u1, …, uN-1 +if and only if constexpr is one of the decl-specifiers of the decl-specifier-seq @@ -1650,23 +1651,9 @@

      7

      [ Example 1: -
      struct S { int i; short s; };
-consteval long f(S s) {
-  long result = 0;
-  template for (auto x : s) {
-    result += x;
-  }
-  return result;
-}
-static_assert(f(S{1, 2}) == 3);
      - — end example ] -
      -

      8

      -
      -[ Example 2: 
      consteval int f(auto const&... Containers) {
   int result = 0;
-  template for (auto const& c : {Containers...}) {
+  template for (auto const& c : {Containers...}) {  // OK, enumerating expansion statement
     result += c[0];
   }
   return result;
@@ -1676,29 +1663,32 @@ 
      static_assert(f(c1, c2) == 5);
      — end example ]
      -

      [ Editor's note: The -following example assumes the changes proposed by P2996R13 and P3491R2. -]

      +

      8

      +
      +[ Example 2: +
      consteval int f() {
+  constexpr std::array<int, 3> arr {1, 2, 3};
+  int result = 0;
+  template for (constexpr int s : arr) {  // OK, iterating expansion statement
+    result += sizeof(char[s]);
+  }
+  return result;
+}
+static_assert(f() == 6);
      + — end example ] +

      9

      [ Example 3: -
      template <typename T> consteval std::optional<int> f() {
-  constexpr auto statics = std::define_static_array(
-      std::meta::static_data_members_of(
-          ^^T,
-          std::meta::access_context::current()));
-  template for (constexpr std::meta::info s : statics)
-    if constexpr (std::meta::identifier_of(s) == "ClsId")
-      return [:s:];
-  return std::nullopt;
-}
-
-struct Cls {
-  static constexpr int ClsId = 14;
-  static constexpr int Stuff[] = {1, 2};
-};
-
-static_assert(f<Cls>().value() == 14);
      +
      struct S { int i; short s; };
+consteval long f(S s) {
+  long result = 0;
+  template for (auto x : s) {  // OK, destructuring expansion statement
+    result += sizeof(x);
+  }
+  return result;
+}
+static_assert(f(S{}) == sizeof(int) + sizeof(short));
      — end example ]
      @@ -1733,64 +1723,68 @@

      -1 +

      1 A continue statement shall be enclosed by ([stmt.pre]) an iteration-statement ([stmt.iter]) or an expansion-statement -([stmt.expand]). The +([stmt.expand]). If the nearest enclosing such +statement is an +iteration-statement, +the The continue -statement causes control to pass to the loop continuation portion of -the smallest such enclosing statement, that is, to the end of the loop -or expansion. More precisely, in each of the -statements +statement causes control to pass to the loop continuation portion of the smallest +such enclosing statement, that is, to the end of the loop. More +precisely, in each of the statements end +of the statement or +compound-statement of the +nearest enclosing +iteration-statement. +Otherwise, control passes to the end of the +compound-statement of the +current +Si +([stmt.expand]).

      +
      + -
      -
      while (foo) {
-  {
-    // ...
-  }
-contin: ;
-}
      +
      while (foo) {
+  {
+    // ...
+  }
+contin: ;
+}
      		 -
      do {
-  {
-    // ...
-  }
-contin: ;
-} while (foo);
      +
      do {
+  {
+    // ...
+  }
+contin: ;
+} while (foo);
      		 -
      for (;;) {
-  {
-    // ...
-  }
-contin: ;
-}
      -      		 -
      -
      template for (auto e : foo) {
-  {
-    // ...
-  }
-contin: ;
-}
      -
      +
      for (;;) {
+  {
+    // ...
+  }
+contin: ;
+}
      -

      a -continue not -contained in an enclosing iteration or -expansion statement is equivalent to -goto contin.

      + +
      +

      a +continue not +contained in an enclosing iteration statement is equivalent to +goto contin.

      -

      Make a drive-by fix to paragraph 6 of 9.7 [dcl.struct.bind] to +

      Make a drive-by fix to paragraph 6 of 9.6 [dcl.struct.bind] to handle arrays of unknown bound:

      @@ -1807,49 +1801,28 @@

      T.

      -

      Update the fallthrough attribute wording in 9.13.5 [dcl.attr.fallthrough]/1 -to discuss expansion statements:

      -
      -
      -

      1 -The attribute-token -fallthrough may be applied to a null -statement; such a statement is a fallthrough statement. No -attribute-argument-clause -shall be present. A fallthrough statement may only appear within an -enclosing -switch -statement ([stmt.switch]). The next statement that would be executed -after a fallthrough statement shall be a labeled statement whose label -is a case label or default label for the same -switch -statement and, if the fallthrough statement is contained in an iteration -statement or expansion statement, the next -statement shall be part of the same execution of the substatement of the -innermost enclosing iteration statement or the same -expansion of the innermost enclosing expansion statement. The -program is ill-formed if there is no such statement.

      -
      -
      -

      Update the list of templated entities:

      +

      Update the list of templated entities in [temp.pre]/8:

      -

      8 +

      8 An entity is templated if it is

        -
      • (8.1) +
      • (8.1) a template,
        • -
      • (8.2) -an entity defined ([basic.def]) or created ([class.temporary]) in a -templated entity,
        • -
      • (8.3) -an entity defined or created within an expansion statement -([stmt.expand]),
        • -
      • (8.4) +
      • (8.2) +an entity defined ([basic.def]) or created +([class.temporary]) within the +for-range-declaration or +the expansion-statement of +an expansion statement ([stmt.expand]),
        • +
      • (8.3) +an entity defined ([basic.def]) or created ([class.temporary]) +in a templated entity,
        • +
      • (8.4) a member of a templated entity,
        • -
      • (8.5) +
      • (8.5) an enumerator for an enumeration that is a templated entity, or
        • -
      • (8.6) +
      • (8.6) the closure type of a lambda-expression ([expr.prim.lambda.closure]) appearing in the declaration of a templated entity.
      @@ -1860,7 +1833,7 @@

      -

      6 +

      6 The validity of a templated entity may be checked prior to any instantiation.

      [ Note 3: Knowing which @@ -1868,13 +1841,13 @@

      (6.1) +

    • (6.1) no valid specialization, ignoring static_assert-declarations that fail ([dcl.pre]), can be generated for a templated entity or a substatement of a constexpr if statement ([stmt.if]) or expansion statement within a templated entity and the innermost enclosing template is not instantiated, or

      • -

    • (6.2) +

    • (6.2) […]

    @@ -1883,10 +1856,10 @@

    -

    * +

    * For an expansion statement, the point of instantiation immediately -follows the statement of -the +follows the +compound-statement of the expansion-statement.

    @@ -1895,18 +1868,18 @@

    -

    3 +

    3 An id-expression is type-dependent if it is a template-id that is not a concept-id and is dependent; or if its terminal name is

      -
    • (3.1) +
    • (3.1) […]
      • -
    • (3.10) +
    • (3.10) a conversion-function-id that specifies a dependent type, or
      • -
    • (3.10+) +
    • (3.10+) a name introduced by the for-range-declaration D of an expansion statement @@ -1914,15 +1887,15 @@

      D contains a placeholder type and either

      • -
    • (3.11) +
    • (3.11) dependent

    or if it names […]

    @@ -1932,25 +1905,25 @@

    -

    2 +

    2 An id-expression is value-dependent if

      -
    • (2.1) +
    • (2.1) […]
      • -
    • (2.3) +
    • (2.3) it is the name of a constant template parameter,
      • -
    • (2.3+) +
    • (2.3+) it is a name introduced by the for-range-declaration of an expansion statement ([stmt.expand])
      • -
    • (2.4) +
    • (2.4) […]

    4.1 Feature-test-macro

    -

    Add to 15.12 [cpp.predefined]:

    +

    Add to 15.11 [cpp.predefined]:

    diff --git a/1306_expansion_statements/expansion-statements.md b/1306_expansion_statements/expansion-statements.md index a2b0757b..64699e0b 100644 --- a/1306_expansion_statements/expansion-statements.md +++ b/1306_expansion_statements/expansion-statements.md @@ -57,7 +57,7 @@ number of specializations, which can consume significant compilation time and me Third, library-based approaches rely on placing the repeated statements in a lambda body, which changes the semantics of something like a `return` statement — and makes coroutines unusable. -Lastly, "iteration" over destructurable classes effectively requires language support to implement correctly. +Lastly, "iteration" over destructuring classes effectively requires language support to implement correctly. Here are some basic usage examples: @@ -136,7 +136,7 @@ The expansion statement ::: std ```cpp -template for ($init-statement$@~opt~@ $for-range-declaration$ : $expansion-initializer$) $statement$ +template for ($init-statement$@~opt~@ $for-range-declaration$ : $expansion-initializer$) $compound-statement$ ``` ::: @@ -150,19 +150,19 @@ will determine an _expansion size_ based on the `$expansion-initializer$` and th { $for-range-declaration$ = $E$(0); - $statement$ + $compound-statement$ } { $for-range-declaration$ = $E$(1); - $statement$ + $compound-statement$ } // ... repeated up to ... { $for-range-declaration$ = $E$($expansion-size$ - 1); - $statement$ + $compound-statement$ } } @@ -337,7 +337,7 @@ the evaluation of a constant expression (certainly necessary to produce a "`cons some wacky experiments that amounted to trying to "rip the constant evaluator in half" (i.e., separating the "evaluation state", whereby dynamically allocated values are stored, from the rest of the metadata pertaining to an evaluation), we decided to fold: as of the [@P1306R3] revision, we instead propose restricting -expansion over iterable expressions to only cover those that are constant expression. +expansion over expansion-iterable expressions to only cover those that are constant expression. In other words — the desugaring described above (which is similar to the desugaring for the C++11 range-based `for` statement) — is what you get. No special cases. @@ -495,18 +495,18 @@ Update [basic.scope.block]{.sref}/1.1 to include expansion statements: ::: -Modify [class.temporary]{.sref}/5 to clarify that there are now six contexts: +Modify [class.temporary]{.sref}/5 to stop counting contexts: ::: std -[5]{.pnum} There are [five]{.rm} [six]{.addu} contexts in which temporaries are destroyed at a different point than the end of the full-expression. [...] +[5]{.pnum} There are [five]{.rm} [several]{.addu} contexts in which temporaries are destroyed at a different point than the end of the full-expression. [...] ::: Insert a new paragraph after [class.temporary]{.sref}/7 to extend the lifetime of temporaries created by expansion statements, and update the ordinal number used in paragraph 8: ::: std -[7]{.pnum} The fourth context is when a temporary object other than a function parameter object is created in the `$for-range-initializer$` of a range-based `for` statement. [...] +[7]{.pnum} The fourth context is when a temporary object is created in the `$for-range-initializer$` of [either]{.addu} a range-based for statement [or an enumerating expansion statement]{.addu}. If such a temporary object would otherwise be destroyed at the end of the `$for-range-initializer$` `$full-expression$`, the object persists for the lifetime of the reference initialized by the `$for-range-initializer$`. -[[7+]{.pnum} The fifth context is when a temporary object other than a function parameter object is created in the `$expansion-initializer$` of an iterable or destructurable expansion statement, or in a full-expression in the `$expansion-init-list$` of an enumerated expansion statement ([stmt.expand]). If such a temporary object would otherwise be destroyed at the end of that full-expression, the object persists for the lifetime of the reference initialized by the expression in the expanded expansion statement.]{.addu} +[[7+]{.pnum} The fifth context is when a temporary object is created in the `$expansion-initializer$` of an iterating or destructuring expansion statement, or in a full-expression in the `$expansion-init-list$` of an enumerating expansion statement ([stmt.expand]). If such a temporary object would otherwise be destroyed at the end of that `$expansion-initializer$` or full-expression, the object persists for the lifetime of the reference initialized by the `$expansion-initializer$`, if any.]{.addu} [8]{.pnum} The [fifth]{.rm} [sixth]{.addu} context is when a temporary object is created in a structured binding declaration ([dcl.struct.bind]). [...] @@ -540,7 +540,7 @@ Add a production for expansion statements to `$statement$` to [stmt.pre]{.sref}. $structured-binding-declaration$ $initializer$ + $for-range-declaration$: -+ $attribute-specifier-seq@~_opt_~@ $decl-specifier-seq$ $declarator$ ++ $attribute-specifier-seq$@~_opt_~@ $decl-specifier-seq$ $declarator$ + $structured-binding-declaration$ + + $for-range-initializer$: @@ -559,7 +559,7 @@ Extend "substatement" to cover expansion statements in [stmt.pre]{.sref}/2: - [#.#]{.pnum} for a `$compound-statement$`, any `$statement$` of its `$statement-seq$`, - [#.#]{.pnum} for a `$selection-statement$`, any of its `$statement$`s or `$compound-statement$`s (but not its `$init-statement$`), [or]{.rm} - [#.#]{.pnum} for an `$iteration-statement$`, its `$statement$` (but not an `$init-statement$`)[.]{.rm}[, or]{.addu} -- [[#.#]{.pnum} for an `$expansion-statement$`, its `$statement$` (but not an `$init-statement$`).]{.addu} +- [[#.#]{.pnum} for an `$expansion-statement$`, its `$compound-statement$` (but not an `$init-statement$`).]{.addu} ::: @@ -569,7 +569,7 @@ Extend "enclose" to cover expansion statements in [stmt.pre]{.sref}/3: [3]{.pnum} A `$statement$` `S1` _encloses_ a `$statement$` `S2` if - [#.#]{.pnum} `S2` is a substatement of `S1`, -- [#.#]{.pnum} `S1` is a `$selection-statement$`[,]{.addu} [or]{.rm} `$iteration-statement$`[, or `$expansion-statement$`]{.addu} and `S2` is the `$init-statement$` of `S1`, +- [#.#]{.pnum} `S1` is a `$selection-statement$`[,]{.addu} [or]{.rm} `$iteration-statement$`[, or `$expansion-statement$`,]{.addu} and `S2` is the `$init-statement$` of `S1`, - [#.#]{.pnum} [...] ::: @@ -584,7 +584,7 @@ Extend [stmt.pre]{.sref}/8 to cover `$for-range-declaration$`s: Add a new paragraph to the end of [stmt.label]{.sref}: ::: std -[[4]{.pnum} An identifier label shall not occur in an `$expansion-statement$` ([stmt.expand]).]{.addu} +[[4]{.pnum} An identifier label shall not be enclosed by an `$expansion-statement$` ([stmt.expand]).]{.addu} ::: @@ -625,7 +625,7 @@ Insert this section after [stmt.iter]{.sref} (and renumber accordingly). ``` $expansion-statement$: - template for ( $init-statement$@~_opt_~@ $for-range-declaration$ : $expansion-initializer$ ) $statement$ + template for ( $init-statement$@~_opt_~@ $for-range-declaration$ : $expansion-initializer$ ) $compound-statement$ $expansion-initializer$: $expression$ @@ -635,24 +635,24 @@ $expansion-init-list$: { $expression-list$@~_opt_~@ } ``` -[#]{.pnum} The `$statement$` of an `$expansion-statement$` is a control-flow-limited statement ([stmt.label]). +[#]{.pnum} The `$compound-statement$` of an `$expansion-statement$` is a control-flow-limited statement ([stmt.label]). -[#]{.pnum} For the purpose of name lookup and instantiation, the `$for-range-declaration$` and the `$statement$` of the `$expansion-statement$` are together considered a template definition. +[#]{.pnum} For the purpose of name lookup and instantiation, the `$for-range-declaration$` and the `$compound-statement$` of the `$expansion-statement$` are together considered a template definition. -[#]{.pnum} For an expression `$E$`, let the expressions `$begin-expr$` and `$end-expr$` be determined as specified in [stmt.ranged]. An expression is _iterable_ if it does not have array type and either +[#]{.pnum} For an expression `$E$`, let the expressions `$begin-expr$` and `$end-expr$` be determined as specified in [stmt.ranged]. An expression is _expansion-iterable_ if it does not have array type and either - [#.#]{.pnum} `$begin-expr$` and `$end-expr$` are of the form `$E$.begin()` and `$E$.end()` or -- [#.#]{.pnum} argument-dependent lookup for `begin($E$)` and `end($E$)` find at least one function or function template for each. +- [#.#]{.pnum} argument-dependent lookups for `begin($E$)` and for `end($E$)` each find at least one function or function template. [#]{.pnum} An expansion statement is -* [#.#]{.pnum} an _enumerated expansion statement_ if its `$expansion-initializer$` is of the form `$expansion-init-list$`; -* [#.#]{.pnum} an _iterable expansion statement_ if its `$expansion-initializer$` is an iterable expression; -* [#.#]{.pnum} otherwise, it is a _destructurable expansion statement_. +* [#.#]{.pnum} an _enumerating expansion statement_ if its `$expansion-initializer$` is of the form `$expansion-init-list$`; +* [#.#]{.pnum} otherwise, an _iterating expansion statement_ if its `$expansion-initializer$` is an expansion-iterable expression; +* [#.#]{.pnum} otherwise, a _destructuring expansion statement_. -[#]{.pnum} An expansion statement `$S$` is equivalent to a `$compound-statement$` containing instantiations of the `$for-range-declaration$` (including its implied initialization), together with the `$statement$`; these instantiations correspond to expressions resulting from the analysis of the `$expansion-initializer$` as follows: +[#]{.pnum} An expansion statement `$S$` is equivalent to a `$compound-statement$` containing instantiations of the `$for-range-declaration$` (including its implied initialization), together with the `$compound-statement$` of `$S$`, as follows: -- [#.#]{.pnum} If `$S$` is an enumerated expansion statement, `$S$` is equivalent to: +- [#.#]{.pnum} If `$S$` is an enumerating expansion statement, `$S$` is equivalent to: ```cpp { $init-statement$ @@ -662,18 +662,18 @@ $expansion-init-list$: } ``` - where `$S$@~_i_~@` is + where `$N$` is the number of elements in the `$expression-list$`, `$S$@~_i_~@` is ```cpp { $for-range-declaration$ = $E$@~_i_~@ ; - $statement$ + $compound-statement$ } ``` - and `$E$@~_i_~@` is the _i_^th^ `$expression$` in the `$expression-list$`. + and `$E$@~_i_~@` is the _i_^th^ element of the `$expression-list$`. -- [#.#]{.pnum} Otherwise, if `$S$` is an iterable expansion statement, `$S$` is equivalent to: +- [#.#]{.pnum} Otherwise, if `$S$` is an iterating expansion statement, `$S$` is equivalent to: ```cpp { $init-statement$ @@ -681,33 +681,37 @@ $expansion-init-list$: static constexpr auto $begin$ = $begin-expr$; // see [stmt.ranged] static constexpr auto $end$ = $end-expr$; // see [stmt.ranged] - static constexpr std::ptrdiff_t N = [] consteval { - std::ptrdiff_t result = 0; - for (auto i = $begin$; i != $end$; ++i, ++result) ; - return result; - }(); - $S$@~_0_~@ @...@ - $S$@~_N-1_~@ + $S$@~_$N$-1_~@ } ``` - where `$S$@~_i_~@` is + where `$N$` is the result of evaluating the expression + + ```cpp + [] consteval { + std::ptrdiff_t result = 0; + for (auto i = $begin$; i != $end$; ++i, ++result) ; + return result; // distance from begin to end + }() + ``` + + and `$S$@~_i_~@` is ```cpp { - static constexpr auto @*iter*~i~@ = $begin$ + i; - $for-range-declaration$ = *@*iter*~i~@; - $statement$ + static constexpr auto @*iter*@ = $begin$ + i; + $for-range-declaration$ = *@*iter*@; + $compound-statement$ } ``` - for all `i` in the range `[0, $N$)`. The variables `$range$`, `$begin$`, `$end$`, `$N$`, and `@*iter*~i~@` are defined for exposition only. + The variables `$range$`, `$begin$`, `$end$`, and `@*iter*@` are defined for exposition only. [The instantiation is ill-formed if `$range$` is not a constant expression ([expr.const])]{.note} -- [#.#]{.pnum} Otherwise, let `$N$` be the structured binding size of the type of the `$expansion-initializer$`; `$S$` is equivalent to: +- [#.#]{.pnum} Otherwise, `$S$` is a destructuring expansion statement and `$S$` is equivalent to: ```cpp { $init-statement$ @@ -718,30 +722,31 @@ $expansion-init-list$: } ``` - where `$S$@~_i_~@` is + where `$N$` is the structured binding size of the type of the `$expansion-initializer$` and `$S$@~_i_~@` is ```cpp { $for-range-declaration$ = $u$@~_i_~@ ; - $statement$ + $compound-statement$ } ``` - The `constexpr` is present in the declaration of `$seq$` if and only if `constexpr` is one of the `$decl-specifier$`s of the `$decl-specifier-seq$` of the `$for-range-declaration$`. + The keyword `constexpr` is present in the declaration of `u@~0~@, u@~1~@, @...@, u@~_$N$-1_~@` if and only if `constexpr` is one of the `$decl-specifier$`s of the `$decl-specifier-seq$` of the `$for-range-declaration$`. [#]{.pnum} ::: example ```cpp -struct S { int i; short s; }; -consteval long f(S s) { - long result = 0; - template for (auto x : s) { - result += x; +consteval int f(auto const&... Containers) { + int result = 0; + template for (auto const& c : {Containers...}) { // OK, enumerating expansion statement + result += c[0]; } return result; } -static_assert(f(S{1, 2}) == 3); +constexpr int c1[] = {1, 2, 3}; +constexpr int c2[] = {4, 3, 2, 1}; +static_assert(f(c1, c2) == 5); ``` ::: @@ -749,42 +754,31 @@ static_assert(f(S{1, 2}) == 3); ::: example ```cpp -consteval int f(auto const&... Containers) { +consteval int f() { + constexpr std::array arr {1, 2, 3}; int result = 0; - template for (auto const& c : {Containers...}) { - result += c[0]; + template for (constexpr int s : arr) { // OK, iterating expansion statement + result += sizeof(char[s]); } return result; } -constexpr int c1[] = {1, 2, 3}; -constexpr int c2[] = {4, 3, 2, 1}; -static_assert(f(c1, c2) == 5); +static_assert(f() == 6); ``` ::: -[The following example assumes the changes proposed by P2996R13 and P3491R2.]{.ednote} - [#]{.pnum} ::: example ```cpp -template consteval std::optional f() { - constexpr auto statics = std::define_static_array( - std::meta::static_data_members_of( - ^^T, - std::meta::access_context::current())); - template for (constexpr std::meta::info s : statics) - if constexpr (std::meta::identifier_of(s) == "ClsId") - return [:s:]; - return std::nullopt; +struct S { int i; short s; }; +consteval long f(S s) { + long result = 0; + template for (auto x : s) { // OK, destructuring expansion statement + result += sizeof(x); + } + return result; } - -struct Cls { - static constexpr int ClsId = 14; - static constexpr int Stuff[] = {1, 2}; -}; - -static_assert(f().value() == 14); +static_assert(f(S{}) == sizeof(int) + sizeof(short)); ``` ::: @@ -804,7 +798,9 @@ Modify [stmt.cont]{.sref}/1 to allow `continue` in expansion statements: ::: std -[1]{.pnum} A `continue` statement shall be enclosed by ([stmt.pre]) an `$iteration-statement$` ([stmt.iter]) [or an `$expansion-statement$` ([stmt.expand])]{.addu}. The `continue` statement causes control to pass to the [loop]{.rm} continuation portion of the smallest such enclosing statement, that is, to the end of the loop [or expansion]{.addu}. More precisely, in each of the statements +[1]{.pnum} A `continue` statement shall be enclosed by ([stmt.pre]) an `$iteration-statement$` ([stmt.iter]) [or an `$expansion-statement$` ([stmt.expand])]{.addu}. [If the nearest enclosing such statement is an `$iteration-statement$`, the]{.addu} [The]{.rm} `continue` statement causes control to pass to the [loop continuation portion of the smallest such enclosing statement, that is, to the end of the loop. More precisely, in each of the statements]{.rm} [end of the `$statement$` or `$compound-statement$` of the nearest enclosing `$iteration-statement$`.]{.addu} [Otherwise, control passes to the end of the `$compound-statement$` of the current `$S$@~_i_~@` ([stmt.expand]).]{.addu} + +::: rm -
    @@ -840,21 +836,10 @@ contin: ; ``` -::: addu -```cpp -template for (auto e : foo) { - { - // ... - } -contin: ; -} -``` +
    ::: - - -a `continue` not contained in an enclosing iteration [or expansion]{.addu} statement is equivalent to `goto contin`. +[a `continue` not contained in an enclosing iteration statement is equivalent to `goto contin`.]{.rm} ::: @@ -865,21 +850,14 @@ Make a drive-by fix to paragraph 6 of [dcl.struct.bind]{.sref} to handle arrays ::: -Update the fallthrough attribute wording in [dcl.attr.fallthrough]{.sref}/1 to discuss expansion statements: - -::: std -[1]{.pnum} The `$attribute-token$` `fallthrough` may be applied to a null statement; such a statement is a fallthrough statement. No `$attribute-argument-clause$` shall be present. A fallthrough statement may only appear within an enclosing `switch` statement ([stmt.switch]). The next statement that would be executed after a fallthrough statement shall be a labeled statement whose label is a case label or default label for the same `switch` statement and, if the fallthrough statement is contained in an iteration statement [or expansion statement]{.addu}, the next statement shall be part of the same execution of the substatement of the innermost enclosing iteration statement [or the same expansion of the innermost enclosing expansion statement]{.addu}. The program is ill-formed if there is no such statement. - -::: - -Update the list of templated entities: +Update the list of templated entities in [temp.pre]/8: ::: std [8]{.pnum} An entity is _templated_ if it is - [#.#]{.pnum} a template, -- [#.#]{.pnum} an entity defined ([basic.def]) or created ([class.temporary]) in a templated entity, -- [[#.#]{.pnum} an entity defined or created within an expansion statement ([stmt.expand]),]{.addu} +- [[#.#]{.pnum} an entity defined [([basic.def])]{.addu} or created [([class.temporary])]{.addu} within the `$for-range-declaration$` or the `$expansion-statement$` of an expansion statement ([stmt.expand]),]{.addu} +- [#.#]{.pnum} an entity defined [([basic.def])]{.rm} or created [([class.temporary])]{.rm} in a templated entity, - [#.#]{.pnum} a member of a templated entity, - [#.#]{.pnum} an enumerator for an enumeration that is a templated entity, or - [#.#]{.pnum} the closure type of a lambda-expression ([expr.prim.lambda.closure]) appearing in the declaration of a templated entity. @@ -905,7 +883,7 @@ Define the point of instantiation for an expansion statement. ::: std ::: addu -[*]{.pnum} For an expansion statement, the point of instantiation immediately follows the `$statement$` of the `$expansion-statement$`. +[*]{.pnum} For an expansion statement, the point of instantiation immediately follows the `$compound-statement$` of the `$expansion-statement$`. ::: ::: @@ -919,7 +897,7 @@ Add the following case to [temp.dep.expr]{.sref}/3 (and renumber accordingly): - [#.10]{.pnum} a `$conversion-function-id$` that specifies a dependent type, or - [[#.10+]{.pnum} a name introduced by the `$for-range-declaration$` `$D$` of an expansion statement `$S$` if `$D$` contains a placeholder type and either]{.addu} - [[#.10+.#]{.pnum} the `$expansion-initializer$` of `$S$` is type-dependent or]{.addu} - - [[#.10+.#]{.pnum} `$S$` is not an iterable expansion statement.]{.addu} + - [[#.10+.#]{.pnum} `$S$` is not an iterating expansion statement.]{.addu} - [#.11]{.pnum} dependent or if it names [...]