Changes to how match statements interact with enums (#728)

compiler/builders/ast_to_builder.jou

@@ -508,7 +508,22 @@ class AstToBuilder:
 
         if match_stmt->case_underscore != NULL:
             self->build_body(match_stmt->case_underscore)
-        self->builder->jump(done)
+
+        if (
+            match_stmt->case_underscore == NULL
+            and match_stmt->match_obj.types.implicit_cast_type->kind == TypeKind.Enum
+        ):
+            # The one corner case where match statement invokes UB:
+            #   - User is matching over an enum
+            #   - All enum members are handled (otherwise error in typecheck)
+            #   - The value stored in the enum is not a valid value of the enum
+            #   - There is no "case _" to catch the invalid value
+            #
+            # See also: doc/match.md
+            self->builder->unreachable()
+        else:
+            self->builder->jump(done)
+
         self->builder->set_current_block(done)
 
     def build_assert(self, assert_location: Location, assertion: AstAssertion*) -> None:

compiler/typecheck/step3_function_and_method_bodies.jou

@@ -1146,11 +1146,11 @@ def typecheck_match_statement(state: State*, match_stmt: AstMatchStatement*) ->
                 snprintf(msg, sizeof(msg), "case value cannot be <from> when matching with %s", sig_string)
             typecheck_expression_with_implicit_cast(state, case_obj, case_type, msg)
 
-            # The special casing only applies to simple enum member lookups (TheEnum.TheMember syntax)
-            if case_obj->kind != AstExpressionKind.GetEnumMember:
-                nremaining = -1
-
             if nremaining != -1:
+                if case_obj->kind != AstExpressionKind.GetEnumMember:
+                    # Matching an enum but it's too dynamic, not simply TheEnum.Member
+                    snprintf(msg, sizeof(msg), "'case' value must be %s.something when matching a value of enum %s", case_type->name, case_type->name)
+                    fail(case_obj->location, msg)
                 # We are matching against TheEnum.member. Try to find and remove it from remaining members.
                 member = case_obj->enum_member.member_name
                 found = False

tests/other_errors/match_enum_too_dynamic.jou

@@ -0,0 +1,11 @@
+enum Thing:
+    Foo
+    Bar
+    Baz
+
+def do_stuff(t1: Thing, t2: Thing) -> None:
+    match t1:
+        # This is forbidden because it would be hard for the compiler to know
+        # which values have been handled and which haven't
+        case t2:  # Error: 'case' value must be Thing.something when matching a value of enum Thing
+            pass

tests/should_succeed/match.jou

@@ -9,9 +9,9 @@ enum Foo:
     Wut
 
 
-def show_evaluation(foo: Foo, msg: byte*) -> Foo:
+def show_evaluation(value: int, msg: byte*) -> int:
     puts(msg)
-    return foo
+    return value
 
 
 def main() -> int:
@@ -80,6 +80,8 @@ def main() -> int:
             printf("Hey! :)\n")  # Output: Hey! :)
         case Foo.Wut:
             printf("nope\n")
+        case _:
+            printf("Other!!!\n")
 
     f = 12345 as Foo
     match f:
@@ -91,6 +93,8 @@ def main() -> int:
             printf("nope\n")
         case Foo.Wut:
             printf("nope\n")
+        case _:
+            printf("Other!!!\n")  # Output: Other!!!
 
     # Test evaluation order.
     #
@@ -99,25 +103,25 @@ def main() -> int:
     # Output: case 2
     # Output: case 3
     # Output: ye
-    match show_evaluation(Foo.Lol, "match obj"):
-        case show_evaluation(Foo.Bar, "case 1"):
+    match show_evaluation(3, "match obj"):
+        case show_evaluation(1, "case 1"):
             printf("nope\n")
-        case show_evaluation(Foo.Baz, "case 2"):
+        case show_evaluation(2, "case 2"):
             printf("nope\n")
-        case show_evaluation(Foo.Lol, "case 3"):
+        case show_evaluation(3, "case 3"):
             printf("ye\n")
-        case show_evaluation(Foo.Wut, "case 4"):
+        case show_evaluation(4, "case 4"):
             printf("nope\n")
 
     # Output: match obj
     # Output: case 1
     # Output: case 2
     # Output: case 3
     # Output: ye
-    match show_evaluation(Foo.Lol, "match obj"):
-        case show_evaluation(Foo.Bar, "case 1") | show_evaluation(Foo.Baz, "case 2"):
+    match show_evaluation(3, "match obj"):
+        case show_evaluation(1, "case 1") | show_evaluation(2, "case 2"):
             printf("nope\n")
-        case show_evaluation(Foo.Lol, "case 3") | show_evaluation(Foo.Wut, "case 4"):
+        case show_evaluation(3, "case 3") | show_evaluation(4, "case 4"):
             printf("ye\n")
 
     # Make a string that is surely not == "Hello", to make sure strcmp() is called below