Sema: Don't produce spurious diagnostic if the constraint type of an failed to resolve

Author: slavapestov

lib/Sema/TypeCheckType.cpp

@@ -3892,6 +3892,10 @@ TypeResolver::resolveExistentialType(ExistentialTypeRepr *repr,
   if (constraintType->is<ExistentialMetatypeType>())
     return constraintType;
 
+  // If we already failed, don't diagnose again.
+  if (constraintType->hasError())
+    return ErrorType::get(getASTContext());
+
   auto anyStart = repr->getAnyLoc();
   auto anyEnd = Lexer::getLocForEndOfToken(getASTContext().SourceMgr, anyStart);
   if (!constraintType->isExistentialType()) {

test/type/explicit_existential.swift

@@ -225,3 +225,6 @@ func testAnyTypeExpr() {
   let invalid = any P
   test(invalid)
 }
+
+func hasInvalidExistential(_: any DoesNotExistIHope) {}
+// expected-error@-1 {{cannot find type 'DoesNotExistIHope' in scope}}