[CSDiagnostics] Add a diagnostic for same-type requirement failure in opaque return type

Follow-up to #72493

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -2687,6 +2687,9 @@ ERROR(type_does_not_conform_anyobject_decl_owner,none,
 ERROR(type_does_not_conform_in_opaque_return,none,
       "return type of %kind0 requires that %1 %select{conform to %2|be a class type}3",
       (const ValueDecl *, Type, Type, bool))
+ERROR(type_is_not_equal_in_opaque_return,none,
+      "return type of %kind0 requires the types %1 and %2 be equivalent",
+      (const ValueDecl *, Type, Type))
 ERROR(types_not_equal_decl,none,
       "%kind0 requires the types %1 and %2 be equivalent",
       (const ValueDecl *, Type, Type))

lib/Sema/CSDiagnostics.cpp

@@ -455,8 +455,15 @@ bool RequirementFailure::diagnoseAsError() {
 
   if (auto *OTD = dyn_cast<OpaqueTypeDecl>(AffectedDecl)) {
     auto *namingDecl = OTD->getNamingDecl();
-    emitDiagnostic(diag::type_does_not_conform_in_opaque_return,
-                   namingDecl, lhs, rhs, rhs->isAnyObject());
+
+    auto &req = getRequirement();
+    if (req.getKind() == RequirementKind::SameType) {
+      emitDiagnostic(diag::type_is_not_equal_in_opaque_return, namingDecl, lhs,
+                     rhs);
+    } else {
+      emitDiagnostic(diag::type_does_not_conform_in_opaque_return, namingDecl,
+                     lhs, rhs, rhs->isAnyObject());
+    }
 
     if (auto *repr = namingDecl->getOpaqueResultTypeRepr()) {
       emitDiagnosticAt(repr->getLoc(), diag::opaque_return_type_declared_here)

test/type/opaque.swift

@@ -603,11 +603,11 @@ do {
 
   func test1() -> some P3<Int> { // expected-note {{opaque return type declared here}}
     return G<S>()
-    // expected-error@-1 {{type of local function 'test1()' requires that 'S' conform to 'Int'}}
+    // expected-error@-1 {{return type of local function 'test1()' requires the types 'S' and 'Int' be equivalent}}
   }
 
   func test2() -> some P3<G<S>> { // expected-note {{opaque return type declared here}}
     return G<S>()
-    // expected-error@-1 {{return type of local function 'test2()' requires that 'S' conform to 'G<S>'}}
+    // expected-error@-1 {{return type of local function 'test2()' requires the types 'S' and 'G<S>' be equivalent}}
   }
 }

test/type/parameterized_protocol.swift

@@ -158,7 +158,7 @@ struct OpaqueTypes<E> {
   func returnSequenceOfIntBad() -> some Sequence<Int> {
     // expected-note@-1 {{opaque return type declared here}}
     return ConcreteSequence<E>()
-    // expected-error@-1 {{return type of instance method 'returnSequenceOfIntBad()' requires that 'E' conform to 'Int'}}
+    // expected-error@-1 {{return type of instance method 'returnSequenceOfIntBad()' requires the types 'E' and 'Int' be equivalent}}
   }
 
   // Invalid