[Sema][Windows] Don't special-case void* in __unaligned conversions.

As far as I can tell, MSVC allows the relevant conversions for all
pointer types. Found compiling a Windows SDK header.

I've verified that the updated errors in MicrosoftExtensions.cpp match
the ones that MSVC actually emits, except for the one with a FIXME. (Not
sure why this wasn't done for the patch that added the tests.)

To make up for the missing error, add a warning that triggers on
conversions that drop the __unaligned qualfier.

Differential Revision: https://reviews.llvm.org/D120936

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10272,6 +10272,11 @@ def err_openclcxx_placement_new : Error<
 def warn_mig_server_routine_does_not_return_kern_return_t : Warning<
   "'mig_server_routine' attribute only applies to routines that return a kern_return_t">,
   InGroup<IgnoredAttributes>;
+
+def warn_imp_cast_drops_unaligned : Warning<
+  "implicit cast from type %0 to type %1 drops __unaligned qualifier">,
+  InGroup<DiagGroup<"unaligned-qualifier-implicit-cast">>;
+
 } // end of sema category
 
 let CategoryName = "OpenMP Issue" in {

clang/lib/Sema/SemaExprCXX.cpp

@@ -4641,6 +4641,13 @@ Sema::PerformImplicitConversion(Expr *From, QualType ToType,
             From->getType()->getPointeeType().getAddressSpace())
       CK = CK_AddressSpaceConversion;
 
+    if (!isCast(CCK) &&
+        !ToType->getPointeeType().getQualifiers().hasUnaligned() &&
+        From->getType()->getPointeeType().getQualifiers().hasUnaligned()) {
+      Diag(From->getBeginLoc(), diag::warn_imp_cast_drops_unaligned)
+          << InitialFromType << ToType;
+    }
+
     From = ImpCastExprToType(From, ToType.getNonLValueExprType(Context), CK, VK,
                              /*BasePath=*/nullptr, CCK)
                .get();

clang/lib/Sema/SemaOverload.cpp

@@ -3195,9 +3195,8 @@ static bool isQualificationConversionStep(QualType FromType, QualType ToType,
   Qualifiers FromQuals = FromType.getQualifiers();
   Qualifiers ToQuals = ToType.getQualifiers();
 
-  // Ignore __unaligned qualifier if this type is void.
-  if (ToType.getUnqualifiedType()->isVoidType())
-    FromQuals.removeUnaligned();
+  // Ignore __unaligned qualifier.
+  FromQuals.removeUnaligned();
 
   // Objective-C ARC:
   //   Check Objective-C lifetime conversions.

clang/test/SemaCXX/MicrosoftExtensions.cpp

@@ -85,18 +85,22 @@ void test_unaligned() {
   foo_unaligned(p2);
 
   __unaligned B_unaligned *p3 = 0;
-  int p4 = foo_unaligned(p3);
+  int p4 = foo_unaligned(p3); // expected-error {{cannot initialize a variable of type 'int' with an rvalue of type 'void *'}}
+  // expected-warning@-1 {{implicit cast from type '__unaligned B_unaligned *' to type 'B_unaligned *' drops __unaligned qualifier}}
 
-  B_unaligned *p5 = p3; // expected-error {{cannot initialize a variable of type 'B_unaligned *' with an lvalue of type '__unaligned B_unaligned *'}}
+  B_unaligned *p5 = p3;
+  // expected-warning@-1 {{implicit cast from type '__unaligned B_unaligned *' to type 'B_unaligned *' drops __unaligned qualifier}}
 
   __unaligned B_unaligned *p6 = p3;
 
   p1_aligned_type4 = p2_aligned_type4;
-  p2_aligned_type4 = p1_aligned_type4; // expected-error {{assigning to 'int aligned_type4::*' from incompatible type '__unaligned int aligned_type4::*'}}
+  p2_aligned_type4 = p1_aligned_type4;
+  // expected-warning@-1 {{implicit cast from type '__unaligned int aligned_type4::*' to type 'int aligned_type4::*' drops __unaligned qualifier}}
   p3_aligned_type4 = p1_aligned_type4;
 
   __unaligned int a[10];
-  int *b = a; // expected-error {{cannot initialize a variable of type 'int *' with an lvalue of type '__unaligned int[10]'}}
+  int *b = a;
+  // expected-warning@-1 {{implicit cast from type '__unaligned int[10]' to type 'int *' drops __unaligned qualifier}}
 }
 
 // Test from PR27367
@@ -115,13 +119,18 @@ __inline void FreeIDListArray(LPITEMIDLIST *ppidls) {
 // We should accept type conversion of __unaligned to non-__unaligned references
 typedef struct in_addr {
 public:
-  in_addr(in_addr &a) {} // expected-note {{candidate constructor not viable: no known conversion from '__unaligned IN_ADDR *' (aka '__unaligned in_addr *') to 'in_addr &' for 1st argument; dereference the argument with *}}
-  in_addr(in_addr *a) {} // expected-note {{candidate constructor not viable: 1st argument ('__unaligned IN_ADDR *' (aka '__unaligned in_addr *')) would lose __unaligned qualifier}}
+  in_addr(in_addr &a) {} // expected-note {{candidate constructor not viable: expects an lvalue for 1st argument}}
+  in_addr(in_addr *a) {} // expected-note {{candidate constructor not viable: no known conversion from 'IN_ADDR' (aka 'in_addr') to 'in_addr *' for 1st argument}}
 } IN_ADDR;
 
 void f(IN_ADDR __unaligned *a) {
   IN_ADDR local_addr = *a;
-  IN_ADDR local_addr2 = a; // expected-error {{no viable conversion from '__unaligned IN_ADDR *' (aka '__unaligned in_addr *') to 'IN_ADDR' (aka 'in_addr')}}
+  // FIXME: MSVC accepts the following; not sure why clang tries to
+  // copy-construct an in_addr.
+  IN_ADDR local_addr2 = a; // expected-error {{no viable constructor copying variable of type 'IN_ADDR' (aka 'in_addr')}}
+  // expected-warning@-1 {{implicit cast from type '__unaligned IN_ADDR *' (aka '__unaligned in_addr *') to type 'in_addr *' drops __unaligned qualifier}}
+  IN_ADDR local_addr3(a);
+  // expected-warning@-1 {{implicit cast from type '__unaligned IN_ADDR *' (aka '__unaligned in_addr *') to type 'in_addr *' drops __unaligned qualifier}}
 }
 
 template<typename T> void h1(T (__stdcall M::* const )()) { }