[Clang] remove qualifiers in getCanonicalTypeUnqualified (#170271)

It was assumed that since this is a method on Type, there are no quals
present because usually you get a Type by using operator-> on a
QualType. However, quals aren't always stored in the outermost QualType.
For example, if const A is used as a template parameter, the outer
QualType does not have the const flag set. The const actually lives in
the canonical type inside a SubstTemplateTypeParmType. We therefore need
to explicitly remove quals before returning.

Fixes #135273

---

I've been looking at all the call sites as suggested in
#167881 (comment)
to check that their usage of this method is sane. So far, most of the
sites seem to either want unqualified types or not care, so this patch
should handle some spooky edge cases of those. Some of them need more
thinking, which I'll do soon.

Supersedes #167881

Author: rosefromthedead

clang/docs/ReleaseNotes.rst

@@ -589,6 +589,7 @@ Bug Fixes to C++ Support
 - Fix a crash when extracting unavailable member type from alias in template deduction. (#GH165560)
 - Fix incorrect diagnostics for lambdas with init-captures inside braced initializers. (#GH163498)
 - Fixed spurious diagnoses of certain nested lambda expressions. (#GH149121) (#GH156579)
+- Fix the result of ``__is_pointer_interconvertible_base_of`` when arguments are qualified and passed via template parameters. (#GH135273)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/CanonicalType.h

@@ -213,7 +213,8 @@ inline bool operator!=(CanQual<T> x, CanQual<U> y) {
 using CanQualType = CanQual<Type>;
 
 inline CanQualType Type::getCanonicalTypeUnqualified() const {
-  return CanQualType::CreateUnsafe(getCanonicalTypeInternal());
+  return CanQualType::CreateUnsafe(
+      getCanonicalTypeInternal().getUnqualifiedType());
 }
 
 inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,

clang/test/SemaCXX/type-traits.cpp

@@ -1812,6 +1812,11 @@ union Union {};
 union UnionIncomplete;
 struct StructIncomplete; // #StructIncomplete
 
+#if __cplusplus >= 201402L
+template<class _Base, class _Derived>
+inline constexpr bool IsPointerInterconvertibleBaseOfV = __is_pointer_interconvertible_base_of(_Base, _Derived);
+#endif
+
 void is_pointer_interconvertible_base_of(int n)
 {
   static_assert(__is_pointer_interconvertible_base_of(Base, Derived));
@@ -1880,6 +1885,11 @@ void is_pointer_interconvertible_base_of(int n)
   static_assert(!__is_pointer_interconvertible_base_of(void(&)(int), void(&)(char)));
   static_assert(!__is_pointer_interconvertible_base_of(void(*)(int), void(*)(int)));
   static_assert(!__is_pointer_interconvertible_base_of(void(*)(int), void(*)(char)));
+
+#if __cplusplus >= 201402L
+  static_assert(IsPointerInterconvertibleBaseOfV<const Base, Base>);
+  static_assert(IsPointerInterconvertibleBaseOfV<const Base, Derived>);
+#endif
 }
 }