Properly type-check Expression::Select arguments. (#1442)

Require the `accept` and `reject` arguments to `select` to be scalars or
vectors, per WGSL spec.

src/valid/expression.rs

@@ -795,7 +795,14 @@ impl super::Validator {
                     Ti::Scalar {
                         kind: Sk::Bool,
                         width: _,
-                    } => accept_inner.is_sized(),
+                    } => {
+                        // When `condition` is a single boolean, `accept` and
+                        // `reject` can be vectors or scalars.
+                        match *accept_inner {
+                            Ti::Scalar { .. } | Ti::Vector { .. } => true,
+                            _ => false,
+                        }
+                    }
                     Ti::Vector {
                         size,
                         kind: Sk::Bool,

tests/wgsl-errors.rs

@@ -945,3 +945,45 @@ fn invalid_runtime_sized_arrays() {
         if struct_name == "Outer" && member_name == "unsized"
     }
 }
+
+#[test]
+fn select() {
+    check_validation_error! {
+        "
+        fn select_pointers(which: bool) -> i32 {
+            var x: i32 = 1;
+            var y: i32 = 2;
+            let ptr = select(&x, &y, which);
+            return *ptr;
+        }
+        ",
+        "
+        fn select_arrays(which: bool) -> i32 {
+            var x: array<i32, 4>;
+            var y: array<i32, 4>;
+            let s = select(x, y, which);
+            return s[0];
+        }
+        ",
+        "
+        struct S { member: i32; };
+        fn select_structs(which: bool) -> S {
+            var x: S = S(1);
+            var y: S = S(2);
+            let s = select(x, y, which);
+            return s;
+        }
+        ":
+        Err(
+            naga::valid::ValidationError::Function {
+                name,
+                error: naga::valid::FunctionError::Expression {
+                    error: naga::valid::ExpressionError::InvalidSelectTypes,
+                    ..
+                },
+                ..
+            },
+        )
+        if name.starts_with("select_")
+    }
+}