Fix crash when function contains fused extension type (#6204)

Cython/Compiler/PyrexTypes.py

@@ -5042,7 +5042,7 @@ def best_match(arg_types, functions, pos=None, env=None, args=None):
     if len(candidates) == 1:
         return candidates[0][0]
     elif len(candidates) == 0:
-        if pos is not None:
+        if pos is not None and errors:
             func, errmsg = errors[0]
             if len(errors) == 1 or [1 for func, e in errors if e == errmsg]:
                 error(pos, errmsg)

tests/errors/fused_types.pyx

@@ -86,6 +86,14 @@ cdef void contents_unfindable1(cython.integral x):
     z: floating = 1  # note: cdef variables also fail with an error but not by the time this test aborts
     sz = sizeof(floating)
 
+cdef fused fused3:
+    Foo
+    cython.int
+
+cdef cython.integral func_with_fused_extension(fused3 foo):
+    return x
+
+func_with_fused_extension(5)
 
 _ERRORS = u"""
 11:15: fused_type does not take keyword arguments
@@ -112,4 +120,6 @@ _ERRORS = u"""
 87:16: Type cannot be specialized since it is not a fused argument to this function
 87:16: Type cannot be specialized since it is not a fused argument to this function
 87:16: Type cannot be specialized since it is not a fused argument to this function
+93:5: Return type is a fused type that cannot be determined from the function arguments
+96:0: Invalid use of fused types, type cannot be specialized
 """