[GSB] Downgrade "neither type in same-type has a generic param" error to warning.

There is nothing specifically wrong with uttering a same-type
constraint in a where clause where both sides are concrete
types. Downgrade this to a warning; we'll check that the concrete
types match (of course), and such a well-formed constraint will simply
be canonicalized away.

This aids the migration of IndexDistance from an associated type to
Int.

(cherry picked from commit 879b180)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -1686,10 +1686,11 @@ ERROR(requires_not_suitable_archetype,none,
       "type %0 in conformance requirement does not refer to a "
       "generic parameter or associated type",
       (TypeLoc))
-ERROR(requires_no_same_type_archetype,none,
-      "neither type in same-type refers to a generic parameter or "
-      "associated type",
-      ())
+WARNING(requires_no_same_type_archetype,none,
+        "neither type in same-type constraint (%0 or %1) refers to a "
+        "generic parameter  or associated type",
+        (Type, Type))
+
 ERROR(requires_generic_params_made_equal,none,
       "same-type requirement makes generic parameters %0 and %1 equivalent",
       (Type, Type))

lib/AST/GenericSignatureBuilder.cpp

@@ -4131,19 +4131,6 @@ ConstraintResult GenericSignatureBuilder::addSameTypeRequirement(
                                       diagnoseMismatch);
 }
 
-/// Determine whether the given type has a type parameter or directly
-/// references a deprecated typealias.
-static bool hasTypeParameterOrIsDeprecatedTypealias(Type type){
-  if (type->hasTypeParameter()) return true;
-
-  if (auto typeAlias = dyn_cast<NameAliasType>(type.getPointer())) {
-    return typeAlias->getDecl()->getAttrs().getDeprecated(
-                                                      type->getASTContext());
-  }
-
-  return false;
-}
-
 ConstraintResult GenericSignatureBuilder::addSameTypeRequirementDirect(
     ResolvedType type1, ResolvedType type2,
     FloatingRequirementSource source,
@@ -4298,21 +4285,16 @@ ConstraintResult GenericSignatureBuilder::addRequirement(
   }
 
   case RequirementReprKind::SameType: {
-    // Require that at least one side of the requirement contain a type
-    // parameter or directly reference a deprecated declaration.
-    if (!hasTypeParameterOrIsDeprecatedTypealias(Req->getFirstType()) &&
-        !hasTypeParameterOrIsDeprecatedTypealias(Req->getSecondType())) {
-      if (!Req->getFirstType()->hasError() &&
-          !Req->getSecondType()->hasError()) {
-        Impl->HadAnyError = true;
-
-        Diags.diagnose(Req->getEqualLoc(),
-                       diag::requires_no_same_type_archetype)
-          .highlight(Req->getFirstTypeLoc().getSourceRange())
-          .highlight(Req->getSecondTypeLoc().getSourceRange());
-      }
-
-      return ConstraintResult::Concrete;
+    // Warn if neither side of the requirement contains a type parameter.
+    if (!Req->getFirstType()->hasTypeParameter() &&
+        !Req->getSecondType()->hasTypeParameter() &&
+        !Req->getFirstType()->hasError() &&
+        !Req->getSecondType()->hasError()) {
+      Diags.diagnose(Req->getEqualLoc(),
+                     diag::requires_no_same_type_archetype,
+                     Req->getFirstType(), Req->getSecondType())
+        .highlight(Req->getFirstTypeLoc().getSourceRange())
+        .highlight(Req->getSecondTypeLoc().getSourceRange());
     }
 
     auto firstType = subst(Req->getFirstType());

test/Generics/same_type_constraints.swift

@@ -371,3 +371,10 @@ struct StructTakingP1<T: P1> { }
 func resultTypeSuppress<T: P1>() -> StructTakingP1<T> {
   return StructTakingP1()
 }
+
+// Check directly-concrete same-type constraints
+typealias NotAnInt = Double
+
+extension X11 where NotAnInt == Int { }
+// expected-warning@-1{{neither type in same-type constraint ('NotAnInt' (aka 'Double') or 'Int') refers to a generic parameter  or associated type}}
+// expected-error@-2{{generic signature requires types 'NotAnInt' (aka 'Double') and 'Int' to be the same}}

test/decl/typealias/protocol.swift

@@ -268,7 +268,7 @@ extension P10 {
   typealias U = Float
 }
 
-extension P10 where T == Int { } // expected-error{{neither type in same-type refers to a generic parameter or associated type}}
+extension P10 where T == Int { } // expected-warning{{neither type in same-type constraint ('P10.T' (aka 'Int') or 'Int') refers to a generic parameter  or associated type}}
 
 extension P10 where A == X<T> { }
 
@@ -277,3 +277,4 @@ extension P10 where A == X<U> { } // expected-error{{use of undeclared type 'U'}
 extension P10 where A == X<Self.U> { }
 
 extension P10 where V == Int { } // expected-warning 3{{'V' is deprecated: just use Int, silly}}
+// expected-warning@-1{{neither type in same-type constraint ('P10.V' (aka 'Int') or 'Int') refers to a generic parameter  or associated type}}