SIL: Workaround for GenericSignatureBuilder bug

lib/SIL/IR/AbstractionPattern.cpp

@@ -1649,8 +1649,8 @@ class SubstFunctionTypePatternVisitor
       newGPMapping.insert({gp, newParamTy});
       auto substGPTy = Type(gp).subst(substGPMap)->castTo<GenericTypeParamType>();
       substRequirements.push_back(Requirement(RequirementKind::SameType,
-                                              substGPTy,
-                                              newParamTy));
+                                              newParamTy,
+                                              substGPTy));
       assert(!substReplacementTypes[substGPTy->getIndex()]);
       substReplacementTypes[substGPTy->getIndex()] = substParamTy;
     }

test/Generics/rdar86431977.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
+
+protocol P1 {
+  associatedtype A
+  associatedtype B : P1 where B.A == A, B.B == B
+}
+
+protocol P2 : P1 where A == Self {}
+
+struct G<T, U> {}
+
+// The GSB used to get the signature of bar() wrong.
+
+extension G {
+  // CHECK-LABEL: rdar86431977.(file).G extension.foo()@
+  // CHECK: Generic signature: <T, U where T : P2, T == U>
+  func foo() where T : P2, U == T {}
+
+  // CHECK-LABEL: rdar86431977.(file).G extension.bar()@
+  // CHECK: Generic signature: <T, U where T : P2, T == U>
+  func bar() where T : P2, T == U {}
+}

test/SILGen/type_lowering_subst_function_type_requirement_machine.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-emit-silgen %s -requirement-machine-abstract-signatures=verify | %FileCheck %s
+
+struct G<Key: CaseIterable, Value> where Key: RawRepresentable, Value: Codable {
+  var key: Key.RawValue
+}
+
+protocol P: CaseIterable, RawRepresentable {}
+
+struct Value: Codable {}
+
+enum Key: Int, P {
+  case elt
+}
+
+func callee<Key: P>(_: Key.Type, _: @escaping (G<Key, Value>) -> Void) {}
+
+// CHECK-LABEL: sil hidden [ossa] @$s029type_lowering_subst_function_A20_requirement_machine6calleryyF : $@convention(thin) () -> () {
+// CHECK: function_ref @$s029type_lowering_subst_function_A20_requirement_machine6calleryyFyAA1GVyAA3KeyOAA5ValueVGcfU_ : $@convention(thin) @substituted <τ_0_0, τ_0_1, τ_0_2 where τ_0_0 : CaseIterable, τ_0_0 : RawRepresentable, τ_0_0 == τ_0_2, τ_0_1 == Value> (@in_guaranteed G<τ_0_0, Value>) -> () for <Key, Value, Key>
+func caller() {
+  callee(Key.self, { _ in })
+}
+