Don't reject tag declarations in for loop clause-1

[aaron.ballman]

Summary

We currently reject this valid C construct by claiming it declares a non-local variable: for (struct { int i; } s={0}; s.i != 0; s.i--) ;

The problem is that we expect all declaration in the clause-1 declaration statement to be VarDecls of local variables, but there can be other declarations involved such as a tag declaration. We now ignore tag declarations when deciding whether the clause-1 declarations are valid or not. This fixes PR35757.

[aaron.ballman]

Ping

[aaron.ballman]

In D92577#2445714, @aaron.ballman wrote:

Ping

Ping

[rjmccall]

Please test that there's actually an object declared and that it's not *just* a tag declaration.

[aaron.ballman]

Updating based on review feedback.

[aaron.ballman]

In D92577#2458903, @rjmccall wrote:

Please test that there's actually an object declared and that it's not *just* a tag declaration.

Good catch, I've corrected this and added some more test coverage.

[rjmccall]

clang/lib/Sema/SemaStmt.cpp

1846

You can have multiple tag declarations because of complex declarators or type-specifiers, e.g. struct A (*var)(struct B) or _Atomic(struct A(*)(struct B)). It should already be an error to write the latter without a declarator, but you do need to not diagnose if there *is* a declarator. Probably the right way to write this is to ignore tags in the first walk but remember whether you saw a non-tag; if you don't, diagnose the first tag.

[aaron.ballman]

Updating based on review feedback.

clang/lib/Sema/SemaStmt.cpp

1846

Good point on the more complex declarators -- I've added some extra tests for that case as well.

[rjmccall]

LGTM

[aaron.ballman]

Thank you for the review! I've commit in 2d2498ec6c42b12eae873257e6ddce3333fe8348

clang/lib/Sema/SemaStmt.cpp

StmtResult Sema::ActOnForStmt(SourceLocation ForLoc, SourceLocation LParenLoc,
   if (Second.isInvalid())
     return StmtError();
 
   if (!getLangOpts().CPlusPlus) {
     if (DeclStmt *DS = dyn_cast_or_null<DeclStmt>(First)) {
       // C99 6.8.5p3: The declaration part of a 'for' statement shall only
       // declare identifiers for objects having storage class 'auto' or
       // 'register'.
-      for (auto *DI : DS->decls()) {
-        VarDecl *VD = dyn_cast<VarDecl>(DI);
+      for (auto Iter = DS->decl_begin(), End = DS->decl_end(); Iter != End;
+           ++Iter) {
+        VarDecl *VD = dyn_cast<VarDecl>(*Iter);
         if (VD && VD->isLocalVarDecl() && !VD->hasLocalStorage())
           VD = nullptr;
+
         if (!VD) {
-          Diag(DI->getLocation(), diag::err_non_local_variable_decl_in_for);
-          DI->setInvalidDecl();
+          // It is possible to declare non-variable declarations as part of the
+          // declaration part of a 'for' statement, such as defining a structure
+          // or enum type: for (enum { zero, ten } i = zero; i < ten; ++i);
+          if (isa<TagDecl>(*Iter)) {
+            // The declaration immediately after the tag decl needs to be a
+            // variable declaration. We don't care if it's a valid variable
+            // declaration because the validity will be checked on the next
+            // iteration through the declarations in the group.
+            if (Iter + 1 == End || !isa<VarDecl>(*(Iter + 1)))
+              Diag((*Iter)->getLocation(), diag::err_non_variable_decl_in_for);
+          } else {
+            Diag((*Iter)->getLocation(),
+                 diag::err_non_local_variable_decl_in_for);
+            (*Iter)->setInvalidDecl();
+          }
         }
       }
     }
   }
 
   CheckBreakContinueBinding(Second.get().second);
   CheckBreakContinueBinding(third.get());
 

clang/test/Sema/for.c

 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
 // Check C99 6.8.5p3
 void b1 (void) { for (void (*f) (void);;); }
 void b2 (void) { for (void f (void);;); }   // expected-error {{declaration of non-local variable}}
 void b3 (void) { for (static int f;;); }    // expected-error {{declaration of non-local variable}}
 void b4 (void) { for (typedef int f;;); }   // expected-error {{declaration of non-local variable}}
+void b5 (void) { for (struct { int i; } s;;); }
+void b6 (void) { for (enum { zero, ten = 10 } i;;); }
+void b7 (void) { for (struct s { int i; };;); } // expected-error {{non-variable declaration in 'for' loop}}
+void b8 (void) { for (static struct { int i; } s;;); } // expected-error {{declaration of non-local variable}}