Fix test cases/failing/52 inconsistent comparison

Since PR #2884, this is failing with an exception

Keep the behaviour we have had since PR #1810 (0.41.0), that ordering
comparisons of different types fail with an InterpreterException.

Also warn about equality comparisons of different types, which will one day
become an error, as per PR #2884.

mesonbuild/interpreterbase.py

@@ -267,9 +267,8 @@ class InterpreterBase:
 
     def validate_comparison_types(self, val1, val2):
         if type(val1) != type(val2):
-            mlog.warning('''Trying to compare values of different types ({}, {}).
-The result of this is undefined and will become a hard error
-in a future Meson release.'''.format(type(val1).__name__, type(val2).__name__))
+            return False
+        return True
 
     def evaluate_comparison(self, node):
         val1 = self.evaluate_statement(node.left)
@@ -278,11 +277,23 @@ in a future Meson release.'''.format(type(val1).__name__, type(val2).__name__))
         val2 = self.evaluate_statement(node.right)
         if is_disabler(val2):
             return val2
-        self.validate_comparison_types(val1, val2)
+        valid = self.validate_comparison_types(val1, val2)
+        # Ordering comparisons of different types isn't allowed since PR #1810
+        # (0.41.0).  Since PR #2884 we also warn about equality comparisons of
+        # different types, which will one day become an error.
+        if not valid and (node.ctype == '==' or node.ctype == '!='):
+            mlog.warning('''Trying to compare values of different types ({}, {}) using {}.
+The result of this is undefined and will become a hard error
+in a future Meson release.'''.format(type(val1).__name__, type(val2).__name__, node.ctype))
         if node.ctype == '==':
             return val1 == val2
         elif node.ctype == '!=':
             return val1 != val2
+        elif not valid:
+            raise InterpreterException(
+                'Values of different types ({}, {}) cannot be compared using {}.'.format(type(val1).__name__,
+                                                                                         type(val2).__name__,
+                                                                                         node.ctype))
         elif not self.is_elementary_type(val1):
             raise InterpreterException('{} can only be compared for equality.'.format(node.left.value))
         elif not self.is_elementary_type(val2):

test cases/common/19 comparison/meson.build

@@ -126,3 +126,14 @@ test('equalfalse', exe13)
 test('equaltrue', exe14)
 test('nequaltrue', exe15)
 test('nequalfalse', exe16)
+
+# Equality comparisons of different elementary types
+# (these all cause warnings currently, will become an error in future)
+
+assert([] != 'st', 'not equal')
+assert([] != 1, 'not equal')
+assert(2 != 'st', 'not equal')
+
+assert(not ([] == 'st'), 'not equal')
+assert(not ([] == 1), 'not equal')
+assert(not (2 == 'st'), 'not equal')