[Diagnostics] Fix a few issues with existential type mismatches

lib/Sema/CSSimplify.cpp

@@ -8750,7 +8750,27 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
     return SolutionKind::Solved;
   }
 
-  return matchExistentialTypes(type, protocol, kind, flags, locator);
+  auto result = matchExistentialTypes(type, protocol, kind, flags, locator);
+
+  if (shouldAttemptFixes() && result.isFailure()) {
+    auto *loc = getConstraintLocator(locator);
+
+    if (loc->isLastElement<LocatorPathElt::InstanceType>())
+      loc = getConstraintLocator(loc->getAnchor(), loc->getPath().drop_back());
+
+    ConstraintFix *fix = nullptr;
+    if (loc->isLastElement<LocatorPathElt::ApplyArgToParam>()) {
+      fix = AllowArgumentMismatch::create(*this, type, protocol, loc);
+    } else if (loc->isLastElement<LocatorPathElt::ContextualType>()) {
+      fix = ContextualMismatch::create(*this, type, protocol, loc);
+    }
+
+    if (fix) {
+      return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
+    }
+  }
+
+  return result;
 }
 
 void ConstraintSystem::recordSynthesizedConformance(
@@ -9124,6 +9144,13 @@ 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 +9244,7 @@ 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 +9254,7 @@ 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

@@ -120,16 +120,18 @@ func parameterizedExistentials() {
   pt = ppt // expected-error {{cannot assign value of type 'any PP4<Int>.Type' to type 'any P4<Int>.Type'}}
 }
 
+// https://github.com/swiftlang/swift/issues/83991
+
 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) { }
 
-  // Make sure we don't crash.
-  foo(bar(S.self))
+  foo1(bar(S.self)) // expected-error {{argument type 'S.Type' does not conform to expected type 'P'}}
 
-  // FIXME: Bad diagnostic
-  // https://github.com/swiftlang/swift/issues/83991
-  foo(bar(0)) // expected-error {{failed to produce diagnostic for expression}}
+  // Make sure we don't crash.
+  foo2(bar(S.self))
+  foo2(bar(0)) // expected-error {{cannot convert value of type 'Int' to expected argument type 'any P.Type'}}
 }

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'}}
 }