[CSSimplify] Look through InstanceType while attempting to diagnose conformance failures

Matching existential types could introduce `InstanceType` element
at the end of the locator path, it's okay to look through them to
diagnose the underlying issue.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -9124,6 +9124,13 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
         continue;
       }
 
+      // Matching existentials could introduce constraints with `instance type`
+      // element at the end if the confirming type wasn't fully resolved.
+      if (path.back().is<LocatorPathElt::InstanceType>()) {
+        path.pop_back();
+        continue;
+      }
+
       break;
     }
 
@@ -9217,7 +9224,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
       }
     }
 
-    if (loc->isLastElement<LocatorPathElt::MemberRefBase>()) {
+    if (path.back().is<LocatorPathElt::MemberRefBase>()) {
       auto *fix = ContextualMismatch::create(*this, protocolTy, type, loc);
       if (!recordFix(fix))
         return SolutionKind::Solved;
@@ -9227,7 +9234,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
     // for example to `AnyHashable`.
     if ((kind == ConstraintKind::ConformsTo ||
          kind == ConstraintKind::NonisolatedConformsTo) &&
-        loc->isLastElement<LocatorPathElt::ApplyArgToParam>()) {
+        path.back().is<LocatorPathElt::ApplyArgToParam>()) {
       auto *fix = AllowArgumentMismatch::create(*this, type, protocolTy, loc);
       return recordFix(fix, /*impact=*/2) ? SolutionKind::Error
                                           : SolutionKind::Solved;

test/Constraints/existential_metatypes.swift

@@ -124,12 +124,15 @@ func testNestedMetatype() {
   struct S: P {}
 
   func bar<T>(_ x: T) -> T.Type { type(of: x) }
-  func foo(_ x: P.Type.Type) { }
+  func foo1(_ x: P.Type) {}
+  func foo2(_ x: P.Type.Type) { }
+
+  foo1(bar(S.self)) // expected-error {{argument type 'S.Type' does not conform to expected type 'P'}}
 
   // Make sure we don't crash.
-  foo(bar(S.self))
+  foo2(bar(S.self))
 
   // FIXME: Bad diagnostic
   // https://github.com/swiftlang/swift/issues/83991
-  foo(bar(0)) // expected-error {{failed to produce diagnostic for expression}}
+  foo2(bar(0)) // expected-error {{failed to produce diagnostic for expression}}
 }

test/Generics/inverse_generics.swift

@@ -538,5 +538,5 @@ func testYap(_ y: Yapping<NC>) {
 protocol Veggie: ~Copyable {}
 func generalized(_ x: Any.Type) {}
 func testMetatypes(_ t: (any Veggie & ~Copyable).Type) {
-  generalized(t) // expected-error {{cannot convert value of type '(any Veggie & ~Copyable).Type' to expected argument type 'any Any.Type'}}
+  generalized(t) // expected-error {{argument type 'any Veggie & ~Copyable' does not conform to expected type 'Copyable'}}
 }