Fix a bug where fused_to_specific was applied too widely

Fixes https://github.com/cython/cython/issues/3642

Cython/Compiler/Nodes.py

@@ -1023,8 +1023,6 @@ class CSimpleBaseTypeNode(CBaseTypeNode):
                 if scope is None:
                     # Maybe it's a cimport.
                     scope = env.find_imported_module(self.module_path, self.pos)
-                    if scope:
-                        scope.fused_to_specific = env.fused_to_specific
             else:
                 scope = env
 

tests/run/fused_types.pyx

@@ -400,3 +400,26 @@ def test_composite(fused_composite x):
         return x
     else:
         return 2 * x
+
+
+### see GH3642 - presence of cdef inside "unrelated" caused a type to be incorrectly inferred
+cdef unrelated(cython.floating x):
+    cdef cython.floating t
+
+cdef handle_float(float* x): return 'float'
+
+cdef handle_double(double* x): return 'double'
+
+def convert_to_ptr(cython.floating x):
+    """
+    >>> convert_to_ptr(1.0)
+    'double'
+    >>> convert_to_ptr['double'](1.0)
+    'double'
+    >>> convert_to_ptr['float'](1.0)
+    'float'
+    """
+    if cython.floating is float:
+        return handle_float(&x)
+    elif cython.floating is double:
+        return handle_double(&x)