[SILOptimizer] Disable SILCombiner::visitPartialApplyInst for method callees.

lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp

@@ -83,8 +83,10 @@ SILInstruction *SILCombiner::visitPartialApplyInst(PartialApplyInst *PAI) {
     return nullptr;
 
   // partial_apply without any substitutions or arguments is just a
-  // thin_to_thick_function.
-  if (!PAI->hasSubstitutions() && (PAI->getNumArguments() == 0)) {
+  // thin_to_thick_function. thin_to_thick_function supports only thin operands.
+  if (!PAI->hasSubstitutions() && (PAI->getNumArguments() == 0) &&
+      PAI->getSubstCalleeType()->getRepresentation() ==
+          SILFunctionTypeRepresentation::Thin) {
     if (!PAI->isOnStack())
       return Builder.createThinToThickFunction(PAI->getLoc(), PAI->getCallee(),
                                                PAI->getType());

test/AutoDiff/compiler_crashers_fixed/sr12548-siloptimizer-rewrite-partial-apply-convention-method.swift

@@ -0,0 +1,27 @@
+// RUN: %target-build-swift -O %s
+
+// SR-12548: SIL verification error regarding
+// `CapturePropagation::rewritePartialApply` for `partial_apply` with
+// `@convention(method)` callee.
+
+import _Differentiation
+
+protocol Protocol: Differentiable {
+  @differentiable
+  func method() -> Self
+}
+
+extension Protocol {
+  @differentiable
+  func method() -> Self { self }
+}
+
+struct Struct: Protocol {}
+
+let _: @differentiable (Struct) -> Struct = { $0.method() }
+
+// SIL verification failed: operand of thin_to_thick_function must be thin: opFTy->getRepresentation() == SILFunctionType::Representation::Thin
+// Verifying instruction:
+//      // function_ref specialized Protocol.method()
+//   %5 = function_ref @$s7crasher8ProtocolPAAE6methodxyFAA6StructV_TG5 : $@convention(method) (@in_guaranteed Struct) -> @out Struct // user: %6
+// ->   %6 = thin_to_thick_function %5 : $@convention(method) (@in_guaranteed Struct) -> @out Struct to $@callee_guaranteed (@in_guaranteed Struct) -> @out Struct // user: %11

test/SILOptimizer/sil_combine_apply.sil

@@ -974,3 +974,20 @@ bb0(%0: $*FakeProtocol):
   %r = tuple ()
   return %r : $()
 }
+
+// Test `partial_apply` with a `@convention(method)` callee.
+// Rewriting `partial_apply` to `thin_to_thick_function` is not okay because
+// `thin_to_thick_function` supports only `@convention(thin)` operands.
+
+sil @method : $@convention(method) (Int32) -> ()
+
+sil @test_partial_apply_method : $@convention(thin) () -> @owned @callee_owned (Int32) -> () {
+  %1 = function_ref @method : $@convention(method) (Int32) -> ()
+  %2 = partial_apply %1() : $@convention(method) (Int32) -> ()
+  return %2 : $@callee_owned (Int32) -> ()
+}
+
+// CHECK-LABEL: sil @test_partial_apply_method
+// CHECK: [[FN:%.*]] = function_ref @method
+// CHECK-NEXT: partial_apply [[FN]]()
+// CHECK-NEXT: return