[Sema] drop half-baked redundancy checking for now

We already don't diagnose all redundant requirements. Because inverses
can be written in both inheritance and where clauses, but they're not
treated uniformly in the implementation, it's a bit annoying to try and
account for the redundancies in both places; `checkInheritanceClause`
will go over the same "requirements" that we'll also check again in
`swift::rewriting::expandDefaultRequirements`.

Author: kavon

include/swift/AST/DiagnosticsSema.def

@@ -2399,10 +2399,6 @@ ERROR(inferred_opaque_type,none,
 // Inverse Constraints
 ERROR(inverse_type_not_invertible,none,
       "type %0 is not invertible", (Type))
-ERROR(inverse_duplicate,none,
-      "duplicate inverse constraint", ())
-NOTE(inverse_duplicate_previous,none,
-      "previous inverse constraint here", ())
 
 // Extensions
 ERROR(non_nominal_extension,none,

lib/Sema/TypeCheckDecl.cpp

@@ -952,21 +952,9 @@ bool HasNoncopyableAnnotationRequest::evaluate(Evaluator &evaluator, TypeDecl *d
 
   // The set of defaults all types start with.
   auto defaults = InvertibleProtocolSet::full();
-  std::map<InvertibleProtocolKind, SourceLoc> lastInverse;
 
-  // Does correctness checking after inspecting the inheritance clause.
-  // TODO: we can probably move this to `checkInheritanceClause` instead.
   auto foundInverse = [&](SourceLoc loc, InvertibleProtocolKind kind) {
-    // do we still have this default?
-    if (defaults.contains(kind)) {
-      defaults.remove(kind);
-      lastInverse.insert({kind, loc});
-      return;
-    }
-
-    // diagnose duplicate inverses
-    ctx.Diags.diagnose(loc, diag::inverse_duplicate);
-    ctx.Diags.diagnose(lastInverse[kind], diag::inverse_duplicate_previous);
+    defaults.remove(kind);
   };
 
   // Check the inheritance clause for inverses.

test/Generics/inverse_protocols_errors.swift

@@ -41,9 +41,3 @@ struct NCThinger<T: ~Copyable>: ~Copyable, Hello {
   // expected-note@-3 {{add 'inout' for a mutable reference}}
   // expected-note@-4 {{add 'consuming' to take the value from the caller}}
 }
-
-struct ExtraNoncopyStruct: ~Copyable, ~Copyable {}
-// expected-error@-1 {{duplicate inverse constraint}}
-// expected-note@-2 {{previous inverse constraint here}}
-
-protocol ExtraNoncopyProto: ~Copyable, ~Copyable {}

test/Parse/inverses.swift

@@ -24,10 +24,17 @@ func blah<T>(_ t: borrowing T) where T: ~Copyable,
 
 func foo<T: ~Copyable>(x: borrowing T) {}
 
-struct Buurap<T: ~Copyable> {}
+struct Buurap<T: ~Copyable> where T: ~Copyable {}
 
-protocol Foo: ~Copyable  // expected-note {{previous inverse constraint here}}
-         where Self: ~Copyable { // expected-error {{duplicate inverse constraint}}
+struct ExtraNoncopyStruct: ~Copyable, ~Copyable {}
+struct ExtraNoncopyEnum: ~Copyable, ~Copyable {}
+protocol ExtraNoncopyProto: ~Copyable, ~Copyable {}
+
+protocol Foo: ~Copyable
+         where Self: ~Copyable {
+
+    associatedtype Touch : ~Copyable,
+                           ~Copyable
 
     func test<T>(_ t: T) where T: ~Self  // expected-error {{type 'Self' is not invertible}}
 }