[Sema] Check whether `__auto_type` has been deduced before merging

[ahatanak]

Summary

This fixes a bug in clang where it emits the following diagnostic when compiling the test case:

argument to 'sizeof' in 'memset' call is the same pointer type 'S' as the destination

The code that merges __auto_type with other types was committed in https://reviews.llvm.org/D122029.

[aaron.ballman]

Oh, great catch! Thank you for the fix. The changes here LGTM, but precommit CI failed because of patch application. That issue was resolved on the precommit CI machine, so it might not be a bad idea to push up a rebased version of this patch to run precommit CI before landing just to be sure there aren't surprises, or be sure to land it when you're around to react to build failures if any do crop up.

[ahatanak]

Rebase

[ahatanak]

Change the type of memset's length parameter to silence warnings on windows.

clang/lib/AST/ASTContext.cpp

if (OfBlockPointer && !BlockReturnType) {
        if (LHS->isObjCIdType() && RHS->isBlockPointerType())
          return LHS;
       if (RHS->isObjCIdType() && LHS->isBlockPointerType())
         return RHS;
     }
     // Allow __auto_type to match anything; it merges to the type with more
     // information.
     if (const auto *AT = LHS->getAs<AutoType>()) {
-      if (AT->isGNUAutoType())
+      if (!AT->isDeduced() && AT->isGNUAutoType())
         return RHS;
     }
     if (const auto *AT = RHS->getAs<AutoType>()) {
-      if (AT->isGNUAutoType())
+      if (!AT->isDeduced() && AT->isGNUAutoType())
         return LHS;
     }
     return {};
   }
 
   // The canonical type classes match.
   switch (LHSClass) {
 #define TYPE(Class, Base)

clang/test/Sema/warn-memset-bad-sizeof.c

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// expected-no-diagnostics
+
+typedef __SIZE_TYPE__ size_t;
+void *memset(void *, int, size_t);
+
+typedef struct {
+  int a;
+} S;
+
+void test() {
+  S s;
+  __auto_type dstptr = &s;
+  memset(dstptr, 0, sizeof(s));
+}