[sil-combine] Properly form conformances when propagating a concrete type into an existential.

This fixes an optimization bug that resulted in an IRGen crash.

rdar://26286278

Author: swiftix

lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp

@@ -770,9 +770,17 @@ getConformanceAndConcreteType(FullApplySite AI,
   // Find the conformance for the protocol we're interested in.
   for (auto Conformance : Conformances) {
     auto Requirement = Conformance.getRequirement();
-    if (Requirement == Protocol || Requirement->inheritsFrom(Protocol)) {
+    if (Requirement == Protocol) {
       return std::make_pair(Conformance, ConcreteType);
     }
+    if (Requirement->inheritsFrom(Protocol)) {
+      // If Requirement != Protocol, then the abstract conformance cannot be used
+      // as is and we need to create a proper conformance.
+      return std::make_pair(Conformance.isAbstract()
+                                ? ProtocolConformanceRef(Protocol)
+                                : Conformance,
+                            ConcreteType);
+    }
   }
 
   llvm_unreachable("couldn't find matching conformance in substitution?");

test/SILOptimizer/devirt_protocol_method_invocations.swift

@@ -169,3 +169,47 @@ public func testExMetatype() -> Int {
   return type.size
 }
 
+// IRGen used to crash on the testPropagationOfConcreteTypeIntoExistential method.
+// rdar://26286278
+
+protocol MathP {
+  var sum: Int32 { get nonmutating set }
+  func done()
+}
+
+extension MathP {
+  func plus() -> Self {
+    sum += 1
+    return self
+  }
+
+  func minus() {
+    sum -= 1
+    if sum == 0 {
+      done()
+    }
+  }
+}
+
+protocol MathA : MathP {}
+
+public final class V {
+  var a: MathA
+
+  init(a: MathA) {
+    self.a = a
+  }
+}
+
+// Check that all witness_method invocations are devirtualized.
+// CHECK-LABEL: sil @_TTSf4g_d___TF34devirt_protocol_method_invocations44testPropagationOfConcreteTypeIntoExistentialFT1vCS_1V1xVs5Int32_T_
+// CHECK-NOT: witness_method
+// CHECK-NOT: class_method
+// CHECK: return
+public func testPropagationOfConcreteTypeIntoExistential(v: V, x: Int32) {
+  let y = v.a.plus()
+  defer {
+    y.minus()
+  }
+}
+