[AST] Handle classes properly in ProtocolConformanceRef::subst.

Previously ProtocolConformance::subst would crash because it was receiving
things with an unexpected relationship between the conformance's type and the
substituted self type. The compiler doesn't quite properly model "abstract"
inherited conformances, so we end up using normal conformances instead, and we
need to work around this in some cases.

Author: huonw

lib/AST/ProtocolConformance.cpp

@@ -86,9 +86,18 @@ ProtocolConformanceRef::subst(Type origType,
 
   // If we have a concrete conformance, we need to substitute the
   // conformance to apply to the new type.
-  if (isConcrete())
+  if (isConcrete()) {
+    auto concrete = getConcrete();
+    if (auto classDecl = concrete->getType()->getClassOrBoundGenericClass()) {
+      // If this is a class, we need to traffic in the actual type that
+      // implements the protocol, not 'Self' and not any subclasses (with their
+      // inherited conformances).
+      substType =
+          substType->eraseDynamicSelfType()->getSuperclassForDecl(classDecl);
+    }
     return ProtocolConformanceRef(
       getConcrete()->subst(substType, subs, conformances));
+  }
 
   // Opened existentials trivially conform and do not need to go through
   // substitution map lookup.

test/SILOptimizer/devirt_witness_method_conformance.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -O -emit-ir  -primary-file %s | %FileCheck %s
+// This is a swift file because the crash doesn't reproduce with SIL.
+@inline(never)
+func callFoo<T: X>(_ x: T) {
+  x.foo()
+}
+public func a(y: Sub) {
+  callFoo(y)
+  // specialization of callFoo for Sub:
+// CHECK-LABEL: define linkonce_odr hidden swiftcc void @_T033devirt_witness_method_conformance7callFooyxAA1XRzlFAA3SubC_Tg5Tf4d_n() local_unnamed_addr
+}
+protocol X {
+  func foo()
+}
+extension X {
+  func foo() {}
+}
+public class Base: X {}
+public class Sub: Base {}