[CSSimplify] Diagnose an attempt to match non-existential type to an existential one

Resolves: rdar://159401910

Author: xedin

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(

test/Constraints/existential_metatypes.swift

@@ -120,6 +120,8 @@ 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 {}
 
@@ -131,8 +133,5 @@ func testNestedMetatype() {
 
   // Make sure we don't crash.
   foo2(bar(S.self))
-
-  // FIXME: Bad diagnostic
-  // https://github.com/swiftlang/swift/issues/83991
-  foo2(bar(0)) // expected-error {{failed to produce diagnostic for expression}}
+  foo2(bar(0)) // expected-error {{cannot convert value of type 'Int' to expected argument type 'any P.Type'}}
 }