Merge pull request swiftlang#33537 from slavapestov/protocol-requirement-self-where-clause

Sema: Fix crash-on-invalid on protocol requirements with 'where' clause constraints

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -850,7 +850,14 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
   Type openedFullWitnessType;
   Type reqType, openedFullReqType;
 
-  auto reqSig = req->getInnermostDeclContext()->getGenericSignatureOfContext();
+  GenericSignature reqSig = proto->getGenericSignature();
+  if (auto *funcDecl = dyn_cast<AbstractFunctionDecl>(req)) {
+    if (funcDecl->isGeneric())
+      reqSig = funcDecl->getGenericSignature();
+  } else if (auto *subscriptDecl = dyn_cast<SubscriptDecl>(req)) {
+    if (subscriptDecl->isGeneric())
+      reqSig = subscriptDecl->getGenericSignature();
+  }
 
   ClassDecl *covariantSelf = nullptr;
   if (witness->getDeclContext()->getExtendedProtocolDecl()) {

test/decl/protocol/req/unsatisfiable.swift

@@ -58,3 +58,25 @@ protocol P4 {
 protocol P5 {
   associatedtype Y where Y : S // expected-error {{type 'Self.Y' constrained to non-protocol, non-class type 'S'}}
 }
+
+protocol P6 {
+  associatedtype T
+  associatedtype U
+
+  func foo() where T == U
+  // expected-error@-1 {{instance method requirement 'foo()' cannot add constraint 'Self.T == Self.U' on 'Self'}}
+  // expected-note@-2 {{protocol requires function 'foo()' with type '() -> ()'; do you want to add a stub?}}
+}
+
+struct S2 : P6 {
+  // expected-error@-1 {{type 'S2' does not conform to protocol 'P6'}}
+  typealias T = Int
+  typealias U = String
+
+  func foo() {}
+  // expected-note@-1 {{candidate has non-matching type '() -> ()'}}
+
+  // FIXME: This error is bogus and should be omitted on account of the protocol requirement itself
+  // being invalid.
+}
+