[CSSimplify] Handle invalid type specializations

If the base type of the specialization is invalid,
the AST node is going to be replaced with `ErrorExpr`.

We need to handle that gracefully when attempting
to apply specialization in such situations.

Resolves: #77644

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -13936,6 +13936,13 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
   } else {
     // If the overload hasn't been resolved, we can't simplify this constraint.
     auto overloadLocator = getCalleeLocator(getConstraintLocator(locator));
+
+    // If there was a problem resolving specialization expression
+    // it would be diagnosted as invalid AST node.
+    if (overloadLocator->directlyAt<ErrorExpr>()) {
+      return shouldAttemptFixes() ? SolutionKind::Error : SolutionKind::Solved;
+    }
+
     auto selectedOverload = findSelectedOverloadFor(overloadLocator);
     if (!selectedOverload)
       return formUnsolved();

test/Constraints/rdar139913219.swift

@@ -1,14 +1,11 @@
-// RUN: %target-typecheck-verify-swift -verify-ignore-unknown
+// RUN: %target-typecheck-verify-swift
 
 // rdar://139913219 - Make sure we don't crash.
 
 func bar(_ x: Int.Type, _: Int) {}
 func bar<T>(_ x: T.Type, _: Int) {}
 
 func foo() {
-  // FIXME: We shouldn't be failing to produce a diagnostic.
-  // Once resolved, remove '-verify-ignore-unknown'
   bar(X<Int?>.self, .zero)
   // expected-error@-1 {{cannot find 'X' in scope}}
-  // expected-error@-2 {{failed to produce diagnostic for expression}}
 }