[mypyc] Improve error message for multiple inheritance. (#14344)

Ref: mypyc/mypyc#962

Author: jab

mypyc/irbuild/classdef.py

@@ -84,7 +84,7 @@ def transform_class_def(builder: IRBuilder, cdef: ClassDef) -> None:
     # classes aren't necessarily populated yet at
     # prepare_class_def time.
     if any(ir.base_mro[i].base != ir.base_mro[i + 1] for i in range(len(ir.base_mro) - 1)):
-        builder.error("Non-trait MRO must be linear", cdef.line)
+        builder.error("Multiple inheritance is not supported (except for traits)", cdef.line)
 
     if ir.allow_interpreted_subclasses:
         for parent in ir.mro:

mypyc/test-data/commandline.test

@@ -164,7 +164,7 @@ class Trait2(Concrete2):
 class NonExt(Concrete1):  # E: Non-extension classes may not inherit from extension classes
     pass
 
-class Nope(Trait1, Concrete2):  # E: Non-trait bases must appear first in parent list  # E: Non-trait MRO must be linear
+class Nope(Trait1, Concrete2):  # E: Non-trait bases must appear first in parent list  # E: Multiple inheritance is not supported (except for traits)
     pass
 
 @decorator