Sema: Diagnose parametrized protocol types in unsupported contexts

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -3714,6 +3714,8 @@ ERROR(not_a_generic_definition,none,
       "cannot specialize a non-generic definition", ())
 ERROR(not_a_generic_type,none,
       "cannot specialize non-generic type %0", (Type))
+ERROR(parametrized_protocol_not_supported,none,
+      "protocol type with generic argument can only be used as a generic constraint", ())
 ERROR(protocol_does_not_have_primary_assoc_type,none,
       "cannot specialize protocol type %0", (Type))
 ERROR(protocol_cannot_have_multiple_generic_arguments,none,

lib/Sema/TypeCheckType.cpp

@@ -625,9 +625,15 @@ static Type applyGenericArguments(Type type, TypeResolution resolution,
   auto &diags = ctx.Diags;
 
   if (ctx.LangOpts.EnableParametrizedProtocolTypes) {
-    // Build ParametrizedProtocolType if the protocol has a primary associated
-    // type.
     if (auto *protoType = type->getAs<ProtocolType>()) {
+      // Build ParametrizedProtocolType 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().isParametrizedProtocolSupported()) {
+        diags.diagnose(loc, diag::parametrized_protocol_not_supported);
+        return ErrorType::get(ctx);
+      }
+
       auto *protoDecl = protoType->getDecl();
       if (protoDecl->getPrimaryAssociatedType() == nullptr) {
         diags.diagnose(loc, diag::protocol_does_not_have_primary_assoc_type,

lib/Sema/TypeCheckType.h

@@ -250,6 +250,7 @@ class TypeResolutionOptions {
     case Context::TypeAliasDecl:
     case Context::GenericTypeAliasDecl:
     case Context::GenericRequirement:
+    case Context::ExistentialConstraint:
     case Context::MetatypeBase:
       return false;
     case Context::None:
@@ -275,6 +276,40 @@ class TypeResolutionOptions {
     }
   }
 
+  /// Whether parametrized protocol types are supported in this context.
+  bool isParametrizedProtocolSupported() const {
+    switch (context) {
+    case Context::Inherited:
+    case Context::ExtensionBinding:
+    case Context::GenericRequirement:
+      return true;
+    case Context::None:
+    case Context::TypeAliasDecl:
+    case Context::GenericTypeAliasDecl:
+    case Context::MetatypeBase:
+    case Context::ExistentialConstraint:
+    case Context::InExpression:
+    case Context::ExplicitCastExpr:
+    case Context::ForEachStmt:
+    case Context::PatternBindingDecl:
+    case Context::EditorPlaceholderExpr:
+    case Context::ClosureExpr:
+    case Context::FunctionInput:
+    case Context::VariadicFunctionInput:
+    case Context::InoutFunctionInput:
+    case Context::FunctionResult:
+    case Context::SubscriptDecl:
+    case Context::EnumElementDecl:
+    case Context::EnumPatternPayload:
+    case Context::SameTypeRequirement:
+    case Context::ProtocolMetatypeBase:
+    case Context::ImmediateOptionalTypeArgument:
+    case Context::AbstractFunctionDecl:
+    case Context::CustomAttr:
+      return false;
+    }
+  }
+
   /// Determine whether all of the given options are set.
   bool contains(TypeResolutionFlags set) const {
     return !static_cast<bool>(unsigned(set) & ~unsigned(flags));