From f2814caaf04fc041a028d5ede4fb4db5ee2f19ae Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Tue, 24 Oct 2023 05:16:49 +0100
Subject: [PATCH] Sema: don't allow undef values through resolveDefinedValue in
 typeof block

This logic is not correct in most cases. If any instruction needs to
operate with different semantics within `@TypeOf`, it should be made to
do so explicitly.

This broke a line in `std.mem`: I have opted to fix this in std for now,
since as far as I know it's not yet been discussed which operations (if
any) should be special-cased like this within `@TypeOf`.
---
 lib/std/mem.zig | 8 +++++++-
 src/Sema.zig    | 1 -
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/std/mem.zig b/lib/std/mem.zig
index a060db710c..dcd8d1fe1d 100644
--- a/lib/std/mem.zig
+++ b/lib/std/mem.zig
@@ -3666,7 +3666,13 @@ fn ReverseIterator(comptime T: type) type {
         @compileError("expected slice or pointer to array, found '" ++ @typeName(T) ++ "'");
     };
     const Element = std.meta.Elem(Pointer);
-    const ElementPointer = @TypeOf(&@as(Pointer, undefined)[0]);
+    const ElementPointer = @Type(.{ .Pointer = ptr: {
+        var ptr = @typeInfo(Pointer).Pointer;
+        ptr.size = .One;
+        ptr.child = Element;
+        ptr.sentinel = null;
+        break :ptr ptr;
+    } });
     return struct {
         ptr: Pointer,
         index: usize,
diff --git a/src/Sema.zig b/src/Sema.zig
index db54beec09..922ae64861 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -2104,7 +2104,6 @@ fn resolveDefinedValue(
     const mod = sema.mod;
     const val = try sema.resolveValue(air_ref) orelse return null;
     if (val.isUndef(mod)) {
-        if (block.is_typeof) return null;
         return sema.failWithUseOfUndef(block, src);
     }
     return val;