Add a special case to handle "abstract" conformances in IRGen

that GenericSignature minimization resolves as concrete.

rdar://25695389

Author: rjmccall

lib/IRGen/GenArchetype.cpp

@@ -180,6 +180,20 @@ class ArchetypeTypeInfoBase {
     auto wtable = IGF.tryGetLocalTypeData(archetype, localDataKind);
     if (wtable) return wtable;
 
+    // It can happen with class constraints that Sema will consider a
+    // constraint to be abstract, but the minimized signature will
+    // eliminate it as concrete.  Handle this by performing a concrete
+    // lookup.
+    // TODO: maybe Sema shouldn't ever do this?
+    if (Type classBound = archetype->getSuperclass()) {
+      auto conformance =
+        IGF.IGM.getSwiftModule()->lookupConformance(classBound, protocol,
+                                                    nullptr);
+      if (conformance && conformance->isConcrete()) {
+        return emitWitnessTableRef(IGF, archetype, *conformance);
+      }
+    }
+
     // If that's not present, this conformance must be implied by some
     // associated-type relationship.
     auto accessPath =

test/IRGen/class_constraint.sil

@@ -0,0 +1,26 @@
+// RUN: %target-swift-frontend -emit-ir -primary-file %s | FileCheck %s
+
+protocol P {}
+class A : P {}
+
+sil_vtable A {}
+
+// <rdar://25695389>
+//   Ensure that an "abstract" conformance that minimization
+//   resolves as concrete doesn't mess up IRGen.  Note that this test case
+//   is only meaningful if type-checking actually resolves the conformance
+//   as abstract, which it does today for both Swift and SIL inputs.
+//   If it stops doing that, we can drop the special case in IRGen.
+sil @foo : $@convention(thin) <T where T : A, T : P> (@guaranteed T) -> () {
+bb0(%0 : $T):
+  %1 = alloc_stack $P
+  %2 = init_existential_addr %1 : $*P, $T
+  strong_retain %0 : $T
+  store %0 to %2 : $*T
+  destroy_addr %1 : $*P
+  dealloc_stack %1 : $*P
+  %3 = tuple ()
+  return %3 : $()
+}
+// CHECK: define {{.*}}void @foo(%C16class_constraint1A*, %swift.type* %T)
+// CHECK: store i8** @_TWPC16class_constraint1AS_1PS_,