[C] Fix issue with -Wimplicit-void-ptr-cast (llvm#154351)

The changes from llvm#136855 missed
a change with atomic assignment constraints. This fixes a bug where we'd
accidentally drop a non-atomic-to-atomic conversion step.

Fixes llvm#154157

co-authored-by: @ahatanak
(cherry picked from commit ae434cd)

Author: AaronBallman

clang/lib/Sema/SemaExpr.cpp

@@ -9309,14 +9309,14 @@ AssignConvertType Sema::CheckAssignmentConstraints(QualType LHSType,
   // If we have an atomic type, try a non-atomic assignment, then just add an
   // atomic qualification step.
   if (const AtomicType *AtomicTy = dyn_cast<AtomicType>(LHSType)) {
-    AssignConvertType result =
+    AssignConvertType Result =
         CheckAssignmentConstraints(AtomicTy->getValueType(), RHS, Kind);
-    if (result != AssignConvertType::Compatible)
-      return result;
+    if (!IsAssignConvertCompatible(Result))
+      return Result;
     if (Kind != CK_NoOp && ConvertRHS)
       RHS = ImpCastExprToType(RHS.get(), AtomicTy->getValueType(), Kind);
     Kind = CK_NonAtomicToAtomic;
-    return AssignConvertType::Compatible;
+    return Result;
   }
 
   // If the left-hand side is a reference type, then we are in a

clang/test/Sema/implicit-void-ptr-cast.c

@@ -82,3 +82,15 @@ void more(void) {
   ptr3 = SOMETHING_THAT_IS_NOT_NULL; // c-warning {{implicit conversion when assigning to 'char *' from type 'void *' is not permitted in C++}} \
                                         cxx-error {{assigning to 'char *' from incompatible type 'void *'}}
 }
+
+void gh154157(void) {
+  #define ATOMIC_VAR_INIT(value) (value)
+
+  typedef const struct T * T_Ref;
+  static T_Ref _Atomic x = ATOMIC_VAR_INIT((void*)NULL); // c-warning {{implicit conversion when initializing '_Atomic(T_Ref)' with an expression of type 'void *' is not permitted in C++}} \
+                                                            cxx-error {{cannot initialize a variable of type '_Atomic(T_Ref)' with an rvalue of type 'void *'}}
+  static T_Ref const y = ATOMIC_VAR_INIT((void*)NULL);   // c-warning {{implicit conversion when initializing 'const T_Ref' (aka 'const struct T *const') with an expression of type 'void *' is not permitted in C++}} \
+                                                            cxx-error {{cannot initialize a variable of type 'const T_Ref' (aka 'const T *const') with an rvalue of type 'void *'}}
+  static T_Ref z = ATOMIC_VAR_INIT((void*)NULL);         // c-warning {{implicit conversion when initializing 'T_Ref' (aka 'const struct T *') with an expression of type 'void *' is not permitted in C++}} \
+                                                            cxx-error {{cannot initialize a variable of type 'T_Ref' (aka 'const T *') with an rvalue of type 'void *'}}
+}