[C] Disallow declarations where a statement is required (#92908)

This fixes a regression introduced in
8bd06d5b65845e5e01dd899a2deb773580460b89 where Clang began to accept a
declaration where a statement is required. e.g.,
```
if (1)
  int x; // Previously accepted, now properly rejected
```

Fixes #92775

clang/docs/ReleaseNotes.rst

@@ -632,6 +632,9 @@ Bug Fixes in This Version
 - ``__is_array`` and ``__is_bounded_array`` no longer return ``true`` for
   zero-sized arrays. Fixes (#GH54705).
 
+- Correctly reject declarations where a statement is required in C.
+  Fixes #GH92775
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Parse/Parser.h

@@ -467,15 +467,18 @@ class Parser : public CodeCompletionHandler {
 
   /// Flags describing a context in which we're parsing a statement.
   enum class ParsedStmtContext {
+    /// This context permits declarations in language modes where declarations
+    /// are not statements.
+    AllowDeclarationsInC = 0x1,
     /// This context permits standalone OpenMP directives.
-    AllowStandaloneOpenMPDirectives = 0x1,
+    AllowStandaloneOpenMPDirectives = 0x2,
     /// This context is at the top level of a GNU statement expression.
-    InStmtExpr = 0x2,
+    InStmtExpr = 0x4,
 
     /// The context of a regular substatement.
     SubStmt = 0,
     /// The context of a compound-statement.
-    Compound = AllowStandaloneOpenMPDirectives,
+    Compound = AllowDeclarationsInC | AllowStandaloneOpenMPDirectives,
 
     LLVM_MARK_AS_BITMASK_ENUM(InStmtExpr)
   };

clang/lib/Parse/ParseStmt.cpp

@@ -239,7 +239,15 @@ Retry:
     auto IsStmtAttr = [](ParsedAttr &Attr) { return Attr.isStmtAttr(); };
     bool AllAttrsAreStmtAttrs = llvm::all_of(CXX11Attrs, IsStmtAttr) &&
                                 llvm::all_of(GNUAttrs, IsStmtAttr);
-    if (((GNUAttributeLoc.isValid() && !(HaveAttrs && AllAttrsAreStmtAttrs)) ||
+    // In C, the grammar production for statement (C23 6.8.1p1) does not allow
+    // for declarations, which is different from C++ (C++23 [stmt.pre]p1). So
+    // in C++, we always allow a declaration, but in C we need to check whether
+    // we're in a statement context that allows declarations. e.g., in C, the
+    // following is invalid: if (1) int x;
+    if ((getLangOpts().CPlusPlus || getLangOpts().MicrosoftExt ||
+         (StmtCtx & ParsedStmtContext::AllowDeclarationsInC) !=
+             ParsedStmtContext()) &&
+        ((GNUAttributeLoc.isValid() && !(HaveAttrs && AllAttrsAreStmtAttrs)) ||
          isDeclarationStatement())) {
       SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
       DeclGroupPtrTy Decl;

clang/test/C/C99/block-scopes.c

@@ -18,8 +18,9 @@
 
 enum {a, b};
 void different(void) {
-  if (sizeof(enum {b, a}) != sizeof(int))
+  if (sizeof(enum {b, a}) != sizeof(int)) {
     _Static_assert(a == 1, "");
+  }
   /* In C89, the 'b' found here would have been from the enum declaration in
    * the controlling expression of the selection statement, not from the global
    * declaration. In C99 and later, that enumeration is scoped to the 'if'

clang/test/Parser/decls.c

@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic
+
+// Test that we can parse declarations at global scope.
+int v;
+
+void func(void) {
+  // Test that we can parse declarations within a compound statement.
+  int a;
+  {
+    int b;
+  }
+
+  int z = ({ // expected-warning {{use of GNU statement expression extension}}
+	// Test that we can parse declarations within a GNU statement expression.
+	int w = 12;
+	w;
+  });
+
+  // Test that we diagnose declarations where a statement is required.
+  // See GH92775.
+  if (1)
+    int x; // expected-error {{expected expression}}
+  for (;;)
+    int c; // expected-error {{expected expression}}
+
+  label:
+    int y; // expected-warning {{label followed by a declaration is a C23 extension}}
+
+  // Test that lookup works as expected.
+  (void)a;
+  (void)v;
+  (void)z;
+  (void)b; // expected-error {{use of undeclared identifier 'b'}}
+  (void)w; // expected-error {{use of undeclared identifier 'w'}}
+  (void)x; // expected-error {{use of undeclared identifier 'x'}}
+  (void)c; // expected-error {{use of undeclared identifier 'c'}}
+  (void)y;
+}
+

clang/test/SemaOpenACC/parallel-loc-and-stmt.c

@@ -33,9 +33,11 @@ int foo3;
 
 void func() {
   // FIXME: Should we disallow this on declarations, or consider this to be on
-  // the initialization?
+  // the initialization? This is currently rejected in C because
+  // Parser::ParseOpenACCDirectiveStmt() calls ParseStatement() and passes the
+  // statement context as "SubStmt" which does not allow for a declaration in C.
 #pragma acc parallel
-  int foo;
+  int foo; // expected-error {{expected expression}}
 
 #pragma acc parallel
   {