Improve error message when providing alias with a typeid expression where a type was expected. #2944

releasenotes.md

@@ -16,6 +16,7 @@
 - Add `@constinit` to allow old typedef behaviour.
 - Include actual element count in the error message when the array initializer size does not match the expected size.
 - Add `--print-large-functions` for checking which functions likely dominate the compile time.
+- Improve error message when providing `alias` with a typeid expression where a type was expected. #2944
   
 ### Stdlib changes
 - Summarize sort macros as generic function wrappers to reduce the amount of generated code. #2831

src/compiler/parse_global.c

@@ -2421,8 +2421,12 @@ static inline Decl *parse_alias_type(ParseContext *c, AstId contracts)
 				print_error_at(decl->span, "An identifier may not be aliased with type name, it must start with a lower case letter.");
 			}
 			return poisoned_decl;
+		case EXPR_TERNARY:
+		case EXPR_CALL:
+			PRINT_ERROR_AT(expr, "Expected a type to alias here, if you are providing a constant typeid-expression, e.g. 'FOO > 32 ??? long : int', then you need to wrap the expression in '$typefrom'");
+			return poisoned_decl;
 		default:
-			PRINT_ERROR_HERE("Expected a type to alias here.");
+			PRINT_ERROR_AT(expr, "Expected the name of the type to alias here.");
 			return poisoned_decl;
 	}
 	ASSERT(!tok_is(c, TOKEN_LBRACE));

test/test_suite/define/alias_should_typefrom.c3

@@ -0,0 +1,9 @@
+import std;
+
+const C = 2;
+alias BaseType = C < 32 ??? int : long; // #error: Expected a type to alias here, if you are providing a constant typeid-expression
+
+fn void main()
+{
+
+}