[Sema] Avoid diagnosing overrides for invalid decls

Invalid decls won't match any overrides in the
base class, so avoid diagnosing that they don't
override anything.

Author: hamishknight

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1421,6 +1421,11 @@ bool swift::checkOverrides(ValueDecl *decl) {
     return false;
   }
 
+  // Don't bother checking any further for invalid decls since they won't match
+  // anything.
+  if (decl->isInvalid())
+    return true;
+
   // Set up matching, but bail out if there's nothing to match.
   OverrideMatcher matcher(decl);
   if (!matcher) return false;

test/decl/circularity.swift

@@ -86,10 +86,9 @@ class C3: G1<A>, P {
     // expected-error@-2 {{cannot find type 'A' in scope}}
     // expected-note@-3 2{{through reference here}}
     override func run(a: A) {}
-    // expected-error@-1 {{method does not override any method from its superclass}}
-    // expected-error@-2 {{circular reference}}
-    // expected-note@-3 2 {{through reference here}}
-    // expected-note@-4 {{while resolving type 'A'}}
+    // expected-error@-1 {{circular reference}}
+    // expected-note@-2 2 {{through reference here}}
+    // expected-note@-3 {{while resolving type 'A'}}
 }
 
 // Another case that triggers circular override checking.

test/decl/class/override.swift

@@ -414,3 +414,13 @@ open class OpenDerivedFinal : OpenBase {
 open class OpenDerivedStatic : OpenBase {
   override public static func classMethod() {}
 }
+
+// When override matching an invalid decl, avoid emitting
+// another error saying it doesn't override anything.
+class OverrideTypoBaseClass {
+  func foo(_ x: Int) {}
+}
+class OverrideTypoSubclass: OverrideTypoBaseClass {
+  override func foo(_ x: Itn) {} // expected-error {{cannot find type 'Itn' in scope}}
+}
+