[clang] better error message for while loops outside of control flow

report an error when encountering 'while' token parsing declarator

```
clang/test/Parser/while-loop-outside-function.c:3:1: error: while loop outside of a function
while // expected-error {{while loop outside of a function}}
^
clang/test/Parser/while-loop-outside-function.c:7:1: error: while loop outside of a function
while // expected-error {{while loop outside of a function}}
^
```

Fixes: https://github.com/llvm/llvm-project/issues/34462

Differential Revision: https://reviews.llvm.org/D129573

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -538,6 +538,8 @@ def err_invalid_operator_on_type : Error<
   "cannot use %select{dot|arrow}0 operator on a type">;
 def err_expected_unqualified_id : Error<
   "expected %select{identifier|unqualified-id}0">;
+def err_while_loop_outside_of_a_function : Error<
+  "while loop outside of a function">; 
 def err_brackets_go_after_unqualified_id : Error<
   "brackets are not allowed here; to declare an array, "
   "place the brackets after the %select{identifier|name}0">;

clang/lib/Parse/ParseDecl.cpp

@@ -6344,23 +6344,27 @@ void Parser::ParseDirectDeclarator(Declarator &D) {
            diag::err_expected_member_name_or_semi)
           << (D.getDeclSpec().isEmpty() ? SourceRange()
                                         : D.getDeclSpec().getSourceRange());
-    } else if (getLangOpts().CPlusPlus) {
-      if (Tok.isOneOf(tok::period, tok::arrow))
-        Diag(Tok, diag::err_invalid_operator_on_type) << Tok.is(tok::arrow);
-      else {
-        SourceLocation Loc = D.getCXXScopeSpec().getEndLoc();
-        if (Tok.isAtStartOfLine() && Loc.isValid())
-          Diag(PP.getLocForEndOfToken(Loc), diag::err_expected_unqualified_id)
-              << getLangOpts().CPlusPlus;
-        else
-          Diag(getMissingDeclaratorIdLoc(D, Tok.getLocation()),
-               diag::err_expected_unqualified_id)
-              << getLangOpts().CPlusPlus;
-      }
     } else {
-      Diag(getMissingDeclaratorIdLoc(D, Tok.getLocation()),
-           diag::err_expected_either)
-          << tok::identifier << tok::l_paren;
+      if (Tok.getKind() == tok::TokenKind::kw_while) {
+        Diag(Tok, diag::err_while_loop_outside_of_a_function);
+      } else if (getLangOpts().CPlusPlus) {
+        if (Tok.isOneOf(tok::period, tok::arrow))
+          Diag(Tok, diag::err_invalid_operator_on_type) << Tok.is(tok::arrow);
+        else {
+          SourceLocation Loc = D.getCXXScopeSpec().getEndLoc();
+          if (Tok.isAtStartOfLine() && Loc.isValid())
+            Diag(PP.getLocForEndOfToken(Loc), diag::err_expected_unqualified_id)
+                << getLangOpts().CPlusPlus;
+          else
+            Diag(getMissingDeclaratorIdLoc(D, Tok.getLocation()),
+                 diag::err_expected_unqualified_id)
+                << getLangOpts().CPlusPlus;
+        }
+      } else {
+        Diag(getMissingDeclaratorIdLoc(D, Tok.getLocation()),
+             diag::err_expected_either)
+            << tok::identifier << tok::l_paren;
+      }
     }
     D.SetIdentifier(nullptr, Tok.getLocation());
     D.setInvalidType(true);

clang/test/Parser/while-loop-outside-function.c

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+while // expected-error {{while loop outside of a function}}
+(1) {};
+
+// without semicolon
+while // expected-error {{while loop outside of a function}}
+(1) {}
+
+int overload_return(); // expected-note {{previous declaration is here}}
+
+void overload_return() // expected-error {{conflicting types for 'overload_return'}}
+{ 
+    while(1) {};
+    while(1);
+}
+
+while // expected-error {{while loop outside of a function}}
+(1); 
+
+void correct();
+
+void correct() {
+    while(1) {};
+    while(1);
+}

clang/test/Parser/while-loop-outside-function.cpp

@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+while // expected-error {{while loop outside of a function}}
+(true) {};
+
+// without semicolon
+while // expected-error {{while loop outside of a function}}
+(true) {}
+
+do { // expected-error {{expected unqualified-id}}
+    int some_var = 1;
+    some_var += 3;
+} 
+while // expected-error {{while loop outside of a function}}
+(true); 
+
+void someFunction() {
+    while(true) {};
+}
+
+class SomeClass {
+public:
+    while(true) {} // expected-error {{expected member name or ';' after declaration specifiers}}
+    void some_fn() {
+        while(true) {}
+    }
+};