Sema: Fix isOverrideBasedOnType() to check protocol generic signatures

This avoids feeding invalid type parameters to the Requirement Machine
when a protocol requirement looks similar to a protocol requirement in
the inherited protocol but has an incompatible type.

Fixes https://bugs.swift.org/browse/SR-15826 / rdar://problem/89641535.

Author: slavapestov

lib/Sema/TypeCheckDeclOverride.cpp

@@ -164,25 +164,23 @@ bool swift::isOverrideBasedOnType(const ValueDecl *decl, Type declTy,
   //
   // We can still succeed with a subtype match later in
   // OverrideMatcher::match().
-  if (decl->getDeclContext()->getSelfClassDecl()) {
-    if (auto declCtx = decl->getAsGenericContext()) {
-      auto *parentCtx = parentDecl->getAsGenericContext();
+  if (auto declCtx = decl->getAsGenericContext()) {
+    auto *parentCtx = parentDecl->getAsGenericContext();
 
-      if (declCtx->isGeneric() != parentCtx->isGeneric())
-        return false;
+    if (declCtx->isGeneric() != parentCtx->isGeneric())
+      return false;
 
-      if (declCtx->isGeneric() &&
-          (declCtx->getGenericParams()->size() !=
-           parentCtx->getGenericParams()->size()))
-        return false;
+    if (declCtx->isGeneric() &&
+        (declCtx->getGenericParams()->size() !=
+         parentCtx->getGenericParams()->size()))
+      return false;
 
-      auto &ctx = decl->getASTContext();
-      auto sig = ctx.getOverrideGenericSignature(parentDecl, decl);
-      if (sig &&
-          declCtx->getGenericSignature().getCanonicalSignature() !=
-              sig.getCanonicalSignature()) {
-        return false;
-      }
+    auto &ctx = decl->getASTContext();
+    auto sig = ctx.getOverrideGenericSignature(parentDecl, decl);
+    if (sig &&
+        declCtx->getGenericSignature().getCanonicalSignature() !=
+            sig.getCanonicalSignature()) {
+      return false;
     }
   }
 

test/decl/protocol/override_generic.swift

@@ -0,0 +1,19 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol Base {
+  func foo1<T : P>(_: T, _: T.T)
+  func foo2<T : P>(_: T, _: T.T)
+}
+
+protocol Derived : Base {
+  func foo1<T : P>(_: T, _: T.T)
+  func foo2<T : Q>(_: T, _: T.T)
+}
+
+protocol P {
+  associatedtype T
+}
+
+protocol Q {
+  associatedtype T
+}

validation-test/compiler_crashers_2_fixed/sr15826.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+public struct Observable<T> {}
+
+public protocol BaseVariant: CaseIterable, Equatable {}
+
+public protocol FeatureGate {
+    associatedtype Variant: BaseVariant
+}
+
+public enum FeatureVariantState<T: BaseVariant>: Equatable {}
+
+public protocol BaseGatingProvider {
+    func exposeFeatureVariantState<G: FeatureGate>(for featureGate: G)
+        -> Observable<FeatureVariantState<G.Variant>>
+}
+
+public struct UserFeatureGate<Variant: BaseVariant>: FeatureGate {}
+
+public protocol UserGatingProvider: BaseGatingProvider {
+    func exposeFeatureVariantState<V>(for featureGate: UserFeatureGate<V>)
+    -> Observable<FeatureVariantState<V>>
+}