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[clang][Parser] Warn when the body of expansion statement is not a compound statement #209229

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zwuis merged 5 commits into from
Jul 21, 2026
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[clang][Parser] Warn when the body of expansion statement is not a compound statement #209229

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6858ff8
Warn when the body of expansion statement is not a compound statement
zwuis Jul 13, 2026
fac01ce
Address review comments
zwuis Jul 14, 2026
6e840d8
Move comment to where the variable is used
zwuis Jul 14, 2026
4079c4f
Handle `__attribute__(())`
zwuis Jul 15, 2026
7b10efe
Improve diagnostics
zwuis Jul 16, 2026
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1 change: 1 addition & 0 deletions clang/include/clang/Basic/DiagnosticGroups.td
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Original file line number Diff line number Diff line change
Expand Up @@ -350,6 +350,7 @@ def ExtraTokens : DiagGroup<"extra-tokens">;
def CXX98CompatExtraSemi : DiagGroup<"c++98-compat-extra-semi">;
def CXX11ExtraSemi : DiagGroup<"c++11-extra-semi">;
def EmptyInitStatement : DiagGroup<"empty-init-stmt">;
def ExpansionStmtBody : DiagGroup<"expansion-stmt-missing-braces">;
def ExportUnnamed : DiagGroup<"export-unnamed">;
def ExtraSemiStmt : DiagGroup<"extra-semi-stmt", [EmptyInitStatement]>;
def ExtraSemi : DiagGroup<"extra-semi", [CXX98CompatExtraSemi,
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6 changes: 6 additions & 0 deletions clang/include/clang/Basic/DiagnosticParseKinds.td
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Expand Up @@ -457,6 +457,12 @@ def err_expansion_stmt_requires_cxx2c : Error<
"expansion statements are only supported in C++2c">;
def err_for_template : Error<
"'for template' is invalid; use 'template for' instead">;
def ext_expansion_stmt_body_not_compound_stmt : Extension<
"ISO C++ requires the body of an expansion statement to be a compound statement">,
InGroup<ExpansionStmtBody>;
def ext_expansion_stmt_body_attr : Extension<
"ISO C++ forbids attributes before the compound statement of an expansion statement">,
InGroup<ExpansionStmtBody>;
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cor3ntin marked this conversation as resolved.

def err_expected_case_before_expression: Error<
"expected 'case' keyword before expression">;
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11 changes: 11 additions & 0 deletions clang/lib/Parse/ParseStmt.cpp
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Expand Up @@ -2329,6 +2329,9 @@ StmtResult Parser::ParseForStatement(SourceLocation *TrailingElseLoc,
// the other parts.
getCurScope()->EnterLoopBody(PrecedingLabel);

bool BodyStartsWithAttr = Tok.isOneOf(tok::l_square, tok::kw___attribute);
SourceLocation BodyBeginLoc = Tok.getLocation();

// C99 6.8.5p5 - In C99, the body of the for statement is a scope, even if
// there is no compound stmt. C90 does not have this clause. We only do this
// if the body isn't a compound statement to avoid push/pop in common cases.
Expand Down Expand Up @@ -2379,6 +2382,14 @@ StmtResult Parser::ParseForStatement(SourceLocation *TrailingElseLoc,
return StmtError();
}

// attribute-specifier without attribute (`[[]]`) isn't in AST.
// `__declspec()` is only applied to declarations, so we can ignore it.
if (!isa<CompoundStmt>(Body.get()) || BodyStartsWithAttr)

@AaronBallman AaronBallman Jul 15, 2026

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Whelp, I think this logic is correct but I really don't agree with the design in the standard and think there's another extension hiding in here:

int main() {
  template for (int x : {1}) [[clang::atomic(remote_memory)]] {}
}

That's invalid per the standard because the standard requires a compound-statement and that grammar production cannot have a leading attribute. If WG21 used statement instead, then a leading attribute would be accepted as you'd expect.

I think the diagnostic text will be confusing in that situation because there is a compound statement. That suggests we want to handle attributes with their own diagnostic, but that would mean we'd need to get much better about recognizing attributes instead of looking for a single token.

Ideally, we'd have a CompoundStmt or an AttributedStmt which wraps a CompoundStmt, but there are two scenarios I think that won't cover:

template for (int x : {1}) [[]] { } // Empty attributes don't make an AttributedStmt
template for (int x : {1}) [[unknown]] {} // Unknown attributes don't make an AttributedStmt either

Other test cases to consider would be Objective-C++ or lambdas in C++, so it's not just attributes:

template for (int x : {1}) [obj msg];
template for (int x : {1}) [](){}();

@cor3ntin cor3ntin Jul 15, 2026

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I mean, our logic could actually warn by checking the next token is { - that cover all the cases, i think
https://eel.is/c++draft/stmt.block#nt:compound-statement

@AaronBallman AaronBallman Jul 15, 2026

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Possibly. The cases I was worried about for token lookahead also involved lexer stuff like

template for (int x : {1})
#pragma clang diagnostic ...
{}

and whether anything can usefully produce an annotation token before the compound statement.

@Sirraide Sirraide Jul 15, 2026

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Yes, I also think we should just check if the next token is {; but also, all of these issues really make it seem like the wording should just say statement instead...

whether anything can usefully produce an annotation token before the compound statement.

Code completion maybe?

Diag(BodyBeginLoc,
isa<CompoundStmt>(Body.get()->stripLabelLikeStatements())
? diag::ext_expansion_stmt_body_attr
: diag::ext_expansion_stmt_body_not_compound_stmt);

return Actions.FinishCXXExpansionStmt(ForRangeStmt.get(), Body.get());
}

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47 changes: 47 additions & 0 deletions clang/test/Parser/cxx2c-expansion-statements-non-compound-stmt-body.cpp
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@@ -0,0 +1,47 @@
// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -Wexpansion-stmt-missing-braces -verify
// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -Wpedantic -verify

void f() {
template for (int x : {1})
template for (int y : {1}) // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
; // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
template for (int x : {1})
if (x) // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
;
template for (int x : {1})
switch (x) // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
;
template for (int x : {1})
for (;;) // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
;
template for (int x : {1})
while (x) // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
;
template for (int x : {1})
do // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
;
while (x);
template for (int x : {1})
return; // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
template for (int x : {1})
[] {}(); // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
template for (int x : {1})
[ // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
[]] if (x)
;
template for (int x : {1})
[ // expected-warning {{ISO C++ requires the body of an expansion statement to be a compound statement}}
[likely]] if (x)
;
template for (int x : {1})
[ // expected-warning {{ISO C++ forbids attributes before the compound statement of an expansion statement}}
[]] {}
template for (int x : {1})
[ // expected-warning {{ISO C++ forbids attributes before the compound statement of an expansion statement}}
[likely]] {}
template for (int x : {1})
__attribute__ // expected-warning {{ISO C++ forbids attributes before the compound statement of an expansion statement}}
(()) {}
template for (int x : {1})
foo: {} // expected-error {{labels are not allowed in expansion statements}}
}
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