[Sema] Insert ErrorType same-type constraints for placeholder signatures

This helps avoid producing more downstream errors. This changes
`GenericSignature::forInvalid` to produce the same signature as e.g
`<T where T == Undefined>`. This subsumes the need to introduce
conformance requirements for invertible protocols.

lib/AST/ASTContext.cpp

@@ -6012,16 +6012,16 @@ GenericSignature::forInvalid(ArrayRef<GenericTypeParamType *> params) {
   ASSERT(!params.empty());
   auto &ctx = params.front()->getASTContext();
 
+  // Add same type requirements that make each of the generic parameters
+  // concrete error types. This helps avoid downstream diagnostics and is
+  // handled the same as if the user wrote e.g `<T where T == Undefined>`.
   SmallVector<Requirement, 2> requirements;
   for (auto *param : params) {
     if (param->isValue())
       continue;
 
-    for (auto ip : InvertibleProtocolSet::allKnown()) {
-      auto *proto = ctx.getProtocol(getKnownProtocolKind(ip));
-      requirements.emplace_back(RequirementKind::Conformance, param,
-                                proto->getDeclaredInterfaceType());
-    }
+    requirements.emplace_back(RequirementKind::SameType, param,
+                              ErrorType::get(ctx));
   }
   return GenericSignature::get(params, requirements);
 }

lib/AST/Decl.cpp

@@ -1643,8 +1643,7 @@ bool GenericContext::isComputingGenericSignature() const {
 
 /// If we hit a cycle while building the generic signature, we can't return
 /// nullptr, since this breaks invariants elsewhere. Instead, build a dummy
-/// signature where everything is Copyable and Escapable, to avoid spurious
-/// downstream diagnostics concerning move-only types.
+/// invalid signature to avoid spurious downstream diagnostics.
 static GenericSignature getPlaceholderGenericSignature(
     ASTContext &ctx, const DeclContext *DC) {
   SmallVector<GenericParamList *, 2> gpLists;

test/Constraints/same_types.swift

@@ -282,15 +282,11 @@ protocol P2 {
 }
 
 // CHECK-LABEL: same_types.(file).structuralSameTypeRecursive1@
-// CHECK-NEXT: Generic signature: <T, U>
+// CHECK-NEXT: Generic signature: <T, U where T == <<error type>>, U == <<error type>>>
 
 // expected-error@+2 {{cannot build rewrite system for generic signature; concrete type nesting limit exceeded}}
 // expected-note@+1 {{τ_0_0.[P2:Assoc1].[concrete: ((((((((((((((((((((((((((((((((τ_0_0.[P2:Assoc1], τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1), τ_0_1)] => τ_0_0.[P2:Assoc1]}}
-func structuralSameTypeRecursive1<T: P2, U>(_: T, _: U)
-  where T.Assoc1 == Tuple2<T.Assoc1, U>
-  // expected-error@-1 {{'Assoc1' is not a member type of type 'T'}}
-  // expected-error@-2 {{'Assoc1' is not a member type of type 'T'}}
-{ }
+func structuralSameTypeRecursive1<T: P2, U>(_: T, _: U) where T.Assoc1 == Tuple2<T.Assoc1, U> {}
 
 protocol P3 {
 }

test/attr/attr_specialize.swift

@@ -55,8 +55,6 @@ class G<T> {
   @_specialize(where T == S<T>)
   // expected-error@-1 {{cannot build rewrite system for generic signature; concrete type nesting limit exceeded}}
   // expected-note@-2 {{failed rewrite rule is τ_0_0.[concrete: S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<τ_0_0>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>] => τ_0_0}}
-  // expected-error@-3 {{too few generic parameters are specified in '_specialize' attribute (got 0, but expected 1)}}
-  // expected-note@-4 {{missing constraint for 'T' in '_specialize' attribute}}
   @_specialize(where T == Int, U == Int) // expected-error{{cannot find type 'U' in scope}}
 
   func noGenericParams() {}

test/decl/protocol/req/recursion.swift

@@ -11,10 +11,8 @@ extension SomeProtocol where T == Optional<T> { }
 // rdar://problem/19840527
 
 class X<T> where T == X {
-// expected-error@-1 {{cannot build rewrite system for generic signature; concrete type nesting limit exceeded}}
-// expected-note@-2 {{failed rewrite rule is τ_0_0.[concrete: X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<X<τ_0_0>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>] => τ_0_0}}
-// expected-error@-3 {{generic class 'X' has self-referential generic requirements}}
-    var type: T { return Swift.type(of: self) } // expected-error{{cannot convert return expression of type 'X<T>.Type' to return type 'T'}}
+// expected-error@-1 {{generic class 'X' has self-referential generic requirements}}
+    var type: T { return Swift.type(of: self) }
 }
 
 // FIXME: The "associated type 'Foo' is not a member type of 'Self'" diagnostic

validation-test/compiler_crashers_2_fixed/0161-issue-49119.swift

@@ -13,5 +13,4 @@ extension Type: P where Param: P, Param.A == Type<Param> {
   // expected-error@-3 {{type 'Type<Param>' does not conform to protocol 'P'}}
   // expected-note@-4 {{add stubs for conformance}}
   typealias A = Param
-  // expected-note@-1 {{possibly intended match 'Type<Param>.A' (aka 'Param') does not conform to 'P'}}
 }

validation-test/compiler_crashers_2_fixed/issue-52031.swift

@@ -13,9 +13,6 @@ extension S: P where N: P {
   static func f<X: P>(_ x: X) -> S<X.A> where A == X, X.A == N {
   // expected-error@-1 {{cannot build rewrite system for generic signature; rule length limit exceeded}}
   // expected-note@-2 {{τ_0_0.[P:A].[P:A].[P:A].[P:A].[P:A].[concrete: S<S<S<S<S<S<τ_0_0>>>>>>] => τ_0_0.[P:A].[P:A].[P:A].[P:A].[P:A] [subst↓]}}
-  // expected-error@-3 {{'A' is not a member type of type 'X'}}
-  // expected-error@-4 {{'A' is not a member type of type 'X'}}
     return S<X.A>()
-    // expected-error@-1 {{'A' is not a member type of type 'X'}}
   }
 }