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

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -457,6 +457,9 @@ 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_missing_braces : Extension<
+  "ISO C++ requires a compound statement to be the body of an expansion statement">,
+  InGroup<DiagGroup<"expansion-stmt-missing-braces">>;
 
 def err_expected_case_before_expression: Error<
   "expected 'case' keyword before expression">;

clang/lib/Parse/ParseStmt.cpp

@@ -2329,6 +2329,10 @@ StmtResult Parser::ParseForStatement(SourceLocation *TrailingElseLoc,
   // the other parts.
   getCurScope()->EnterLoopBody(PrecedingLabel);
 
+  // attribute-specifier without attribute (`[[]]`) isn't in AST.
+  bool BodyStartsWithSquare = Tok.is(tok::l_square);
+  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.
@@ -2379,6 +2383,9 @@ StmtResult Parser::ParseForStatement(SourceLocation *TrailingElseLoc,
       return StmtError();
     }
 
+    if (!isa<CompoundStmt>(Body.get()) || BodyStartsWithSquare)
+      Diag(BodyBeginLoc, diag::ext_expansion_stmt_missing_braces);
+
     return Actions.FinishCXXExpansionStmt(ForRangeStmt.get(), Body.get());
   }
 

clang/test/Parser/cxx2c-expansion-statements-non-compound-stmt-body.cpp

@@ -0,0 +1,31 @@
+// 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 a compound statement to be the body of an expansion statement}}
+      ; // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+  template for (int x : {1})
+    if (x) // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+      ;
+  template for (int x : {1})
+    switch (x) // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+      ;
+  template for (int x : {1})
+    for (;;) // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+      ;
+  template for (int x : {1})
+    while (x) // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+      ;
+  template for (int x : {1})
+    do // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+      ;
+    while (x);
+  template for (int x : {1})
+    return; // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+  template for (int x : {1})
+    [ // expected-warning {{ISO C++ requires a compound statement to be the body of an expansion statement}}
+      []] {}
+  template for (int x : {1})
+    foo: {} // expected-error {{labels are not allowed in expansion statements}}
+}