SILGen: Use lowered function type parameters in objc-to-swift thunks for initializers.

Fixes a regression caused by the `self` declaration being given a `DynamicSelfType`
even though the formal type of the enclosing function sometimes still uses the
concrete base class type.

Fixes swiftlang#83876 | rdar://158956768.

Author: jckarter

lib/SILGen/SILGenBridging.cpp

@@ -2170,23 +2170,22 @@ void SILGenFunction::emitForeignToNativeThunk(SILDeclRef thunk) {
   SmallVector<SILValue, 8> params;
 
   bindParametersForForwarding(fd->getParameters(), params);
-  if (thunk.kind != SILDeclRef::Kind::Allocator)
-    if (auto *selfDecl = fd->getImplicitSelfDecl())
-      bindParameterForForwarding(selfDecl, params);
-
-  // For allocating constructors, 'self' is a metatype, not the 'self' value
-  // formally present in the constructor body.
   Type allocatorSelfType;
-  if (thunk.kind == SILDeclRef::Kind::Allocator) {
-    auto *selfDecl = fd->getImplicitSelfDecl();
-    allocatorSelfType = F.mapTypeIntoContext(
-      fd->getDeclContext()->getSelfInterfaceType());
-
-    auto selfMetatype =
-      CanMetatypeType::get(allocatorSelfType->getCanonicalType());
-    auto selfArg = F.begin()->createFunctionArgument(
-        getLoweredLoadableType(selfMetatype), selfDecl);
+  if (auto *selfDecl = fd->getImplicitSelfDecl()) {
+    // The self declaration sometimes differ in dynamic-self-ness from the
+    // lowered function type, so get the self parameter type from the function
+    // type rather than the declaration.
+    auto selfArgTy = F.getLoweredFunctionType()->getSelfParameter()
+      .getSILStorageType(getModule(), F.getLoweredFunctionType(), getTypeExpansionContext());
+    auto selfArg = F.begin()->createFunctionArgument(F.mapTypeIntoContext(selfArgTy), selfDecl);
     params.push_back(selfArg);
+
+    // For allocating constructors, 'self' is a metatype, not the 'self' value
+    // formally present in the constructor body.
+    if (thunk.kind == SILDeclRef::Kind::Allocator) {
+      allocatorSelfType = F.mapTypeIntoContext(
+        fd->getDeclContext()->getSelfInterfaceType());
+    }
   }
 
   // Set up the throw destination if necessary.

test/SILGen/Inputs/protocol-static-reqt-objc-class-impl.h

@@ -0,0 +1,7 @@
+@import Foundation;
+
+@interface C: NSObject
+
++(_Nonnull instancetype)foo;
+
+@end

test/SILGen/protocol-static-reqt-objc-class-impl.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) -import-objc-header %S/Inputs/protocol-static-reqt-objc-class-impl.h %s -verify
+// REQUIRES: objc_interop
+
+protocol P {
+	static func foo() -> Self
+}
+
+extension C: P {}
+