fix: Detect unsupported default arguments (#659)

Fixes #658

guppylang/checker/func_checker.py

@@ -176,6 +176,10 @@ def check_signature(func_def: ast.FunctionDef, globals: Globals) -> FunctionType
         raise GuppyError(UnsupportedError(func_def.args.vararg, "Variadic args"))
     if func_def.args.kwarg is not None:
         raise GuppyError(UnsupportedError(func_def.args.kwarg, "Keyword args"))
+    if func_def.args.defaults:
+        raise GuppyError(
+            UnsupportedError(func_def.args.defaults[0], "Default arguments")
+        )
     if func_def.returns is None:
         err = MissingReturnAnnotationError(func_def)
         # TODO: Error location is incorrect

tests/error/misc_errors/default_arg.err

@@ -0,0 +1,8 @@
+Error: Unsupported (at $FILE:5:26)
+  | 
+3 | 
+4 | @compile_guppy
+5 | def foo(x: int, y: bool = True) -> int:
+  |                           ^^^^ Default arguments are not supported
+
+Guppy compilation failed due to 1 previous error

tests/error/misc_errors/default_arg.py

@@ -0,0 +1,6 @@
+from tests.util import compile_guppy
+
+
+@compile_guppy
+def foo(x: int, y: bool = True) -> int:
+    return x