Sema: Better diagnostic when parameterized protocol is used in an unsupported context

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -3788,8 +3788,6 @@ ERROR(protocol_declares_unknown_primary_assoc_type,none,
       (Identifier, Type))
 ERROR(protocol_declares_duplicate_primary_assoc_type,none,
       "duplicate primary associated type name %0", (Identifier))
-ERROR(parameterized_protocol_not_supported,none,
-      "protocol type with type arguments can only be used as a generic constraint", ())
 ERROR(protocol_does_not_have_primary_assoc_type,none,
       "cannot specialize protocol type %0", (Type))
 ERROR(parameterized_protocol_type_argument_count_mismatch,none,

lib/Sema/TypeCheckType.cpp

@@ -629,15 +629,8 @@ static Type applyGenericArguments(Type type, TypeResolution resolution,
   auto &diags = ctx.Diags;
 
   if (auto *protoType = type->getAs<ProtocolType>()) {
-    // Build ParameterizedProtocolType if the protocol has a primary associated
-    // type and we're in a supported context (for now just generic requirements,
-    // inheritance clause, extension binding).
-    if (!resolution.getOptions().isParameterizedProtocolSupported()) {
-      diags.diagnose(loc, diag::parameterized_protocol_not_supported);
-      return ErrorType::get(ctx);
-    }
-
     auto *protoDecl = protoType->getDecl();
+
     auto assocTypes = protoDecl->getPrimaryAssociatedTypes();
     if (assocTypes.empty()) {
       diags.diagnose(loc, diag::protocol_does_not_have_primary_assoc_type,
@@ -657,6 +650,18 @@ static Type applyGenericArguments(Type type, TypeResolution resolution,
       return ErrorType::get(ctx);
     }
 
+    // Build ParameterizedProtocolType if the protocol has a primary associated
+    // type and we're in a supported context (for now just generic requirements,
+    // inheritance clause, extension binding).
+    if (!resolution.getOptions().isParameterizedProtocolSupported()) {
+      diags.diagnose(loc, diag::existential_requires_any,
+                     protoDecl->getDeclaredInterfaceType(),
+                     protoDecl->getExistentialType(),
+                     /*isAlias=*/isa<TypeAliasType>(type.getPointer()));
+
+      return ErrorType::get(ctx);
+    }
+
     // Disallow opaque types anywhere in the structure of the generic arguments
     // to a parameterized existential type.
     if (options.is(TypeResolverContext::ExistentialConstraint))

test/type/parameterized_existential.swift

@@ -7,10 +7,10 @@ protocol Sequence<Element> { // expected-note {{'Sequence' declared here}}
 // 'any' is required here
 
 func takesSequenceOfInt1(_: Sequence<Int>) {}
-// expected-error@-1 {{protocol type with type arguments can only be used as a generic constraint}}
+// expected-error@-1 {{use of protocol 'Sequence' as a type must be written 'any Sequence'}}
 
 func returnsSequenceOfInt1() -> Sequence<Int> {}
-// expected-error@-1 {{protocol type with type arguments can only be used as a generic constraint}}
+// expected-error@-1 {{use of protocol 'Sequence' as a type must be written 'any Sequence'}}
 
 struct ConcreteSequence<Element> : Sequence {}
 
@@ -83,7 +83,7 @@ func splay(_ x: some Pair<Int, String>) -> (Int, String) { fatalError() }
 
 func typeExpr() {
   _ = Sequence<Int>.self
-  // expected-error@-1 {{protocol type with type arguments can only be used as a generic constraint}}
+  // expected-error@-1 {{use of protocol 'Sequence' as a type must be written 'any Sequence'}}
 
   _ = any Sequence<Int>.self
   // expected-error@-1 {{'self' is not a member type of protocol 'parameterized_existential.Sequence<Swift.Int>'}}