AstGen: emit `as` instructions for branching expressions

There is a mechanism to avoid redundant `as` ZIR instructions which is
to pass `ResultLoc.coerced_ty` instead of `ResultLoc.ty` when it is
known by AstGen that Sema will do the coercion.

This commit downgrades `coerced_ty` to `ty` when a result location
passes through an expression that branches, such as `if`, `switch`,
`while`, and `for`, causing the `as` ZIR instruction to be emitted.

This ensures that the type of a result location will be applied to, e.g.
a `comptime_int` on either side of a branch on a runtime condition.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/behavior/array_llvm.zig b/test/behavior/array_llvm.zig
index 5bbb06cffe..b4b0f1f268 100644
--- a/test/behavior/array_llvm.zig
+++ b/test/behavior/array_llvm.zig
@@ -179,3 +179,17 @@ test "access the null element of a null terminated array" {
     try S.doTheTest();
     comptime try S.doTheTest();
 }
+
+test "type deduction for array subscript expression" {
+    const S = struct {
+        fn doTheTest() !void {
+            var array = [_]u8{ 0x55, 0xAA };
+            var v0 = true;
+            try expect(@as(u8, 0xAA) == array[if (v0) 1 else 0]);
+            var v1 = false;
+            try expect(@as(u8, 0x55) == array[if (v1) 1 else 0]);
+        }
+    };
+    try S.doTheTest();
+    comptime try S.doTheTest();
+}