Sema: Diagnose retroactive conditional conformances

Passing in the declared interface type to checkConformance() here
masked a silent failure where if the type declaration was generic
and the conformance conditional, the conditional requirement check
would fail. As a result, we did not diagnose the absence of
@retroactive, nor the unnecessary presence of it.

Since we only care about the existence of some conformance, we
can use lookupConformance() instead.

Author: slavapestov

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1653,7 +1653,7 @@ static void diagnoseRetroactiveConformances(
     proto->walkInheritedProtocols([&](ProtocolDecl *decl) {
 
       // Get the original conformance of the extended type to this protocol.
-      auto conformanceRef = ext->getParentModule()->checkConformance(
+      auto conformanceRef = ext->getParentModule()->lookupConformance(
           extendedType, decl);
       if (!conformanceRef.isConcrete()) {
 

test/IRGen/bitwise-copyable-derived-loadRaw.swift

@@ -6,7 +6,7 @@
 
 public protocol MyBitwiseCopyable : _BitwiseCopyable {}
 
-extension SIMD16 : @retroactive MyBitwiseCopyable where Scalar.SIMD16Storage : MyBitwiseCopyable {}
+extension SIMD16 : MyBitwiseCopyable where Scalar.SIMD16Storage : MyBitwiseCopyable {}
 extension UInt8.SIMD16Storage : MyBitwiseCopyable {}
 
 func doit() {

test/Sema/extension_retroactive_conformances.swift

@@ -15,6 +15,9 @@ public struct Sample5 {}
 public struct Sample6 {}
 
 public struct SampleAlreadyConforms: SampleProtocol1 {}
+
+public struct GenericSample1<T> {}
+
 #else
 
 import Library
@@ -81,4 +84,9 @@ extension OriginallyDefinedInLibrary: SampleProtocol1 {} // ok, @_originallyDefi
 
 #endif
 
+// conditional conformances
+extension GenericSample1: SampleProtocol1 where T: SampleProtocol1 {}
+// expected-warning@-1 {{extension declares a conformance of imported type 'GenericSample1' to imported protocol 'SampleProtocol1'; this will not behave correctly if the owners of 'Library' introduce this conformance in the future}}
+// expected-note@-2 {{add '@retroactive' to silence this warning}}
+
 #endif

test/stdlib/SIMD_as_AdditiveArithmetic.swift

@@ -1,5 +1,5 @@
 // RUN: %target-typecheck-verify-swift
-extension SIMD2: AdditiveArithmetic where Scalar: FloatingPoint { }
-extension SIMD3: AdditiveArithmetic where Scalar: FloatingPoint { }
-extension SIMD4: AdditiveArithmetic where Scalar: FloatingPoint { }
-extension SIMD8: AdditiveArithmetic where Scalar: FloatingPoint { }
+extension SIMD2: @retroactive AdditiveArithmetic where Scalar: FloatingPoint { }
+extension SIMD3: @retroactive AdditiveArithmetic where Scalar: FloatingPoint { }
+extension SIMD4: @retroactive AdditiveArithmetic where Scalar: FloatingPoint { }
+extension SIMD8: @retroactive AdditiveArithmetic where Scalar: FloatingPoint { }