internal/shader: bug fix: treat a built-in function as an invalid argument

Closes #2965

internal/shader/expr.go

@@ -199,7 +199,7 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 				return nil, nil, nil, false
 			}
 			for _, expr := range es {
-				if expr.Type == shaderir.FunctionExpr {
+				if expr.Type == shaderir.FunctionExpr || expr.Type == shaderir.BuiltinFuncExpr {
 					cs.addError(e.Pos(), fmt.Sprintf("function name cannot be an argument: %s", e.Fun))
 					return nil, nil, nil, false
 				}

internal/shader/syntax_test.go

@@ -1897,6 +1897,16 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 	Foo(Bar())
 	return color
 }
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+	// Issue #2965
+	if _, err := compileToIR([]byte(`package main
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	abs(sign)
+	return color
+}
 `)); err == nil {
 		t.Errorf("error must be non-nil but was nil")
 	}