Merge pull request swiftlang#27480 from brentdax/absence-of-results

Fix StringInterpolationProtocol validation crasher

Author: beccadax

lib/AST/Decl.cpp

@@ -6999,11 +6999,8 @@ StaticSpellingKind FuncDecl::getCorrectStaticSpelling() const {
 }
 
 Type FuncDecl::getResultInterfaceType() const {
-  if (!hasInterfaceType())
-    return nullptr;
-
   Type resultTy = getInterfaceType();
-  if (resultTy->is<ErrorType>())
+  if (resultTy.isNull() || resultTy->is<ErrorType>())
     return resultTy;
 
   if (hasImplicitSelfDecl())

validation-test/compiler_crashers_2_fixed/0208-rdar55864759.swift

@@ -0,0 +1,3 @@
+// RUN: not %target-swift-frontend -typecheck %s %S/Inputs/0208-rdar55864759-protocol.swift
+struct StringInterpolation: MagicStringInterpolationProtocol {}
+

validation-test/compiler_crashers_2_fixed/Inputs/0208-rdar55864759-protocol.swift

@@ -0,0 +1,6 @@
+protocol MagicStringInterpolationProtocol: StringInterpolationProtocol {}
+
+extension MagicStringInterpolationProtocol {
+  mutating func appendInterpolation<Value>(_ value: Value) {}
+}
+