fix: Properly report error for unsupported constants (#724)

Fixes #721

guppylang/checker/errors/type_errors.py

@@ -74,7 +74,7 @@ class TypeInferenceError(Error):
 @dataclass(frozen=True)
 class IllegalConstant(Error):
     title: ClassVar[str] = "Unsupported constant"
-    span_label: ClassVar[str] = "Type `{ty}` is not supported"
+    span_label: ClassVar[str] = "Type `{python_ty.__name__}` is not supported"
     python_ty: type
 
 

tests/error/misc_errors/unsupported_const.err

@@ -0,0 +1,8 @@
+Error: Unsupported constant (at $FILE:7:8)
+  | 
+5 | @compile_guppy
+6 | def foo() -> None:
+7 |     x = "foo"
+  |         ^^^^^ Type `str` is not supported
+
+Guppy compilation failed due to 1 previous error

tests/error/misc_errors/unsupported_const.py

@@ -0,0 +1,7 @@
+from guppylang.std.builtins import array
+from tests.util import compile_guppy
+
+
+@compile_guppy
+def foo() -> None:
+    x = "foo"