Improve errors for unresolved TypeForm arguments

mypy/checkexpr.py

@@ -2810,6 +2810,21 @@ def check_arg(
         # Only non-abstract non-protocol class can be given where Type[...] is expected...
         elif self.has_abstract_type_part(caller_type, callee_type):
             self.msg.concrete_only_call(callee_type, context)
+        # Semantic analysis tried and failed to parse this as a type expression.
+        elif (
+            isinstance(context, StrExpr)
+            and context.as_type is None
+            and isinstance(caller_type, Instance)
+            and caller_type.type.fullname == "builtins.str"
+            and isinstance(callee_type, TypeType)
+            and callee_type.is_type_form
+        ):
+            self.chk.fail(
+                "TypeForm containing a string annotation cannot be recognized here. "
+                "Surround with TypeForm(...) to recognize.",
+                context,
+                code=codes.MAYBE_UNRECOGNIZED_STR_TYPEFORM,
+            )
         elif not is_subtype(caller_type, callee_type, options=self.chk.options):
             error = self.msg.incompatible_argument(
                 n,

test-data/unit/check-typeform.test

@@ -157,6 +157,18 @@ is_type('int | None')
 [builtins fixtures/primitives.pyi]
 [typing fixtures/typing-full.pyi]
 
+[case testUnrecognizedStringTypeFormArgument]
+from typing import TypeVar
+from typing_extensions import TypeForm
+T = TypeVar("T")
+def foo(x: TypeForm[T]) -> None: pass
+foo("Unresolvable")  # E: TypeForm containing a string annotation cannot be recognized here. Surround with TypeForm(...) to recognize.
+foo(42)  # E: Argument 1 to "foo" has incompatible type "int"; expected "TypeForm[Never]"
+def foo_or_str(x: TypeForm[T] | str) -> None: pass
+foo_or_str("Unresolvable")
+[builtins fixtures/primitives.pyi]
+[typing fixtures/typing-full.pyi]
+
 
 -- Type Expression Location: Return Statement