FuncParamType should use ValueError message in self.fail()

CHANGES.rst

@@ -49,6 +49,8 @@ Unreleased
     :issue:`3043`
 -   Add :class:`NoSuchCommand` exception with suggestions for misspelled
     commands. :issue:`3107` :pr:`3228`
+-   Use :class:`ValueError` message when conversion in :class:`FuncParamType` would
+    fail. :issue:`3105` :pr:`3211`
 -   Add ``click.get_pager_file`` for file-like access to an output
     pager. :pr:`1572`
 

src/click/types.py

@@ -195,13 +195,16 @@ def convert(
     ) -> ParamTypeValue:
         try:
             return self.func(value)
-        except ValueError:
-            try:
-                value = str(value)
-            except UnicodeError:
-                value = value.decode("utf-8", "replace")
+        except ValueError as exc:
+            message = str(exc)
+
+            if not message:
+                try:
+                    message = str(value)
+                except UnicodeError:
+                    message = value.decode("utf-8", "replace")
 
-            self.fail(value, param, ctx)
+            self.fail(message, param, ctx)
 
 
 class UnprocessedParamType(ParamType[t.Any]):

tests/test_types.py

@@ -51,6 +51,25 @@ def test_range_fail(type, value, expect):
     assert expect in exc_info.value.message
 
 
+@pytest.mark.parametrize(
+    ("error_message", "expected"),
+    [
+        ("bad value: nope", "bad value: nope"),
+        ("", "nope"),
+    ],
+)
+def test_func_param_type_uses_value_error_message(error_message, expected):
+    def parse(value):
+        raise ValueError(error_message if error_message else "")
+
+    func_type = click.types.FuncParamType(parse)
+
+    with pytest.raises(click.BadParameter) as exc_info:
+        func_type.convert("nope", None, None)
+
+    assert expected in exc_info.value.message
+
+
 def test_float_range_no_clamp_open():
     with pytest.raises(TypeError):
         click.FloatRange(0, 1, max_open=True, clamp=True)