Sema: validate that runtime-known inferred alloc does not have comptime-only type

Resolves: #18997

src/Sema.zig

@@ -4149,6 +4149,13 @@ fn zirResolveInferredAlloc(sema: *Sema, block: *Block, inst: Zir.Inst.Index) Com
                 return;
             }
 
+            if (try sema.typeRequiresComptime(final_elem_ty)) {
+                // The alloc wasn't comptime-known per the above logic, so the
+                // type cannot be comptime-only.
+                // TODO: source location of runtime control flow
+                return sema.fail(block, src, "value with comptime-only type '{}' depends on runtime control flow", .{final_elem_ty.fmt(mod)});
+            }
+
             try sema.queueFullTypeResolution(final_elem_ty);
 
             // Change it to a normal alloc.

test/cases/compile_errors/runtime_condition_comptime_type_in_destructure.zig

@@ -0,0 +1,10 @@
+export fn foobar() void {
+    var t = true;
+    _ = &t;
+    const a, _ = if (t) .{ .a, {} } else .{ .b, {} };
+    _ = a;
+}
+
+// error
+//
+// :4:5: error: value with comptime-only type '@TypeOf(.enum_literal)' depends on runtime control flow