shader: Bug fix: a blank identifier cannot be used as values

internal/shader/expr.go

@@ -370,11 +370,17 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr, markLocalVariable
 
 	case *ast.Ident:
 		if e.Name == "_" {
+			// In the context where a local variable is marked as used, any expressions must have its
+			// meaning. Then, a blank identifier is not available there.
+			if markLocalVariableUsed {
+				cs.addError(e.Pos(), fmt.Sprintf("cannot use _ as value"))
+				return nil, nil, nil, false
+			}
 			return []shaderir.Expr{
 				{
 					Type: shaderir.Blank,
 				},
-			}, nil, nil, true
+			}, []shaderir.Type{{}}, nil, true
 		}
 		if i, t, ok := block.findLocalVariable(e.Name, markLocalVariableUsed); ok {
 			return []shaderir.Expr{

internal/shader/stmt.go

@@ -520,11 +520,7 @@ func (cs *compileState) assign(block *block, fname string, pos token.Pos, lhs, r
 					return nil, false
 				}
 
-				var t shaderir.Type
-				if len(ts) == 1 {
-					t = ts[0]
-				}
-				block.addNamedLocalVariable(name, t, e.Pos())
+				block.addNamedLocalVariable(name, ts[0], e.Pos())
 			}
 
 			if len(r) > 1 {

shader_test.go

@@ -563,3 +563,50 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		t.Errorf("error must be non-nil but was nil")
 	}
 }
+
+func TestShaderBlankRhs(t *testing.T) {
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	x := _
+	_ = x
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	var x int = _
+	_ = x
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	x := 1
+	x = _
+	_ = x
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	x := 1 + _
+	_ = x
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+}