Type: check return_type for generic poison before comparing

Closes #13423

Author: Vexu

src/type.zig

@@ -640,7 +640,9 @@ pub const Type = extern union {
                 const a_info = a.fnInfo();
                 const b_info = b.fnInfo();
 
-                if (!eql(a_info.return_type, b_info.return_type, mod))
+                if (a_info.return_type.tag() != .generic_poison and
+                    b_info.return_type.tag() != .generic_poison and
+                    !eql(a_info.return_type, b_info.return_type, mod))
                     return false;
 
                 if (a_info.is_var_args != b_info.is_var_args)

test/behavior/generics.zig

@@ -405,3 +405,15 @@ test "null sentinel pointer passed as generic argument" {
     };
     try S.doTheTest((@intToPtr([*:null]const [*c]const u8, 8)));
 }
+
+test "generic function passed as comptime argument" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+
+    const S = struct {
+        fn doMath(comptime f: fn (type, i32, i32) error{Overflow}!i32, a: i32, b: i32) !void {
+            const result = try f(i32, a, b);
+            try expect(result == 11);
+        }
+    };
+    try S.doMath(std.math.add, 5, 6);
+}