[C2y] Correctly handle incomplete types in generic selections (llvm#141596)

We were emitting a non-error diagnostic but claiming we emitted an
error, which caused some obvious follow-on problems.

Fixes llvm#141549

Author: AaronBallman

clang/lib/Sema/SemaExpr.cpp

@@ -1874,9 +1874,11 @@ ExprResult Sema::CreateGenericSelectionExpr(
 
         if (D != 0) {
           Diag(Types[i]->getTypeLoc().getBeginLoc(), D)
-            << Types[i]->getTypeLoc().getSourceRange()
-            << Types[i]->getType();
-          TypeErrorFound = true;
+              << Types[i]->getTypeLoc().getSourceRange() << Types[i]->getType();
+          if (getDiagnostics().getDiagnosticLevel(
+                  D, Types[i]->getTypeLoc().getBeginLoc()) >=
+              DiagnosticsEngine::Error)
+            TypeErrorFound = true;
         }
 
         // C11 6.5.1.1p2 "No two generic associations in the same generic

clang/test/C/C2y/n3409.c

@@ -1,7 +1,6 @@
-// RUN: %clang_cc1 -verify -std=c2y -pedantic %s
-// RUN: %clang_cc1 -verify=pre-c2y -std=c2y -Wpre-c2y-compat %s
-// RUN: %clang_cc1 -verify=ext -std=c23 -pedantic %s
-// expected-no-diagnostics
+// RUN: %clang_cc1 -verify -std=c2y -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=expected,pre-c2y -std=c2y -Wpre-c2y-compat -Wno-unused %s
+// RUN: %clang_cc1 -verify=expected,ext -std=c23 -pedantic -Wno-unused %s
 
 /* WG14 N3409: Clang 21
  * Slay Some Earthly Demons X
@@ -34,3 +33,25 @@ void foo() {
   // C23 and earlier.
   return x; // ext-warning {{void function 'foo' should not return void expression}}
 }
+
+
+// Ensure we behave correctly with incomplete types. See GH141549.
+static_assert(
+  _Generic(
+    void,    /* ext-warning {{passing a type argument as the first operand to '_Generic' is a C2y extension}}
+                pre-c2y-warning {{passing a type argument as the first operand to '_Generic' is incompatible with C standards before C2y}}
+              */
+    void : 1,
+    default : 0
+  )
+);
+
+static_assert(
+  _Generic( // expected-error {{static assertion failed}}
+    12,
+    void : 1, /* ext-warning {{incomplete type 'void' in a '_Generic' association is a C2y extension}}
+                 pre-c2y-warning {{use of incomplete type 'void' in a '_Generic' association is incompatible with C standards before C2y}}
+               */
+    default : 0
+  )
+);

clang/test/SemaCXX/generic-selection.cpp

@@ -81,7 +81,7 @@ void func(struct S s) {
   // is an elaborated type specifier followed by the association's value and
   // it should work the same as in C.
   (void)_Generic(s, struct S : 1);
-  (void)_Generic(s, struct T : 1);
+  (void)_Generic(s, struct T : 1); // expected-error {{controlling expression type 'struct S' not compatible with any generic association type}}
 
   // The rest of these cases test that we still produce a reasonable diagnostic
   // when referencing an unknown type or trying to define a type in other ways.