[mypyc] Raise "non-trait base must be first..." error less frequently (#14468)

It would raise even if there were only non-trait bases, leading to this
slightly confusing situation:

class A: pass
class B: pass
class C(A, B): pass # E: Non-trait bases must appear first in parent
list
# E: Multiple inheritance is not supported (except for traits)

Now the bases must include a non-trait *and* the first base must be a
trait to error. This leads to some false-negatives when there's more
than one non-trait base, but in that case, it's better to only tell the
user that multiple inheritance is not supported.

See also:
mypyc/mypyc#826 (comment)

Author: ichard26

mypyc/irbuild/prepare.py

@@ -275,8 +275,12 @@ def prepare_class_def(
 
     # Set up the parent class
     bases = [mapper.type_to_ir[base.type] for base in info.bases if base.type in mapper.type_to_ir]
-    if not all(c.is_trait for c in bases[1:]):
-        errors.error("Non-trait bases must appear first in parent list", path, cdef.line)
+    if len(bases) > 1 and any(not c.is_trait for c in bases) and bases[0].is_trait:
+        # If the first base is a non-trait, don't ever error here. While it is correct
+        # to error if a trait comes before the next non-trait base (e.g. non-trait, trait,
+        # non-trait), it's pointless, confusing noise from the bigger issue: multiple
+        # inheritance is *not* supported.
+        errors.error("Non-trait base must appear first in parent list", path, cdef.line)
     ir.traits = [c for c in bases if c.is_trait]
 
     mro = []  # All mypyc base classes

mypyc/test-data/commandline.test

@@ -150,7 +150,7 @@ class PureTrait:
     pass
 
 @trait
-class Trait1(Concrete1):
+class Trait1:
     pass
 
 class Concrete2:
@@ -164,9 +164,23 @@ 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: Multiple inheritance is not supported (except for traits)
+
+class NopeMultipleInheritance(Concrete1, Concrete2):  # E: Multiple inheritance is not supported (except for traits)
+    pass
+
+class NopeMultipleInheritanceAndBadOrder(Concrete1, Trait1, Concrete2):  # E: Multiple inheritance is not supported (except for traits)
+    pass
+
+class NopeMultipleInheritanceAndBadOrder2(Concrete1, Concrete2, Trait1):  # E: Multiple inheritance is not supported (except for traits)
     pass
 
+class NopeMultipleInheritanceAndBadOrder3(Trait1, Concrete1, Concrete2):  # E: Non-trait base must appear first in parent list # E: Multiple inheritance is not supported (except for traits)
+    pass
+
+class NopeBadOrder(Trait1, Concrete2):  # E: Non-trait base must appear first in parent list
+    pass
+
+
 @decorator
 class NonExt2:
     @property  # E: Property setters not supported in non-extension classes