internal/shader: bug fix: error on a function name in a function call

Updates #2891

internal/shader/expr.go

@@ -187,6 +187,12 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 				cs.addError(e.Pos(), fmt.Sprintf("single-value context and multiple-value context cannot be mixed: %s", e.Fun))
 				return nil, nil, nil, false
 			}
+			for _, expr := range es {
+				if expr.Type == shaderir.FunctionExpr {
+					cs.addError(e.Pos(), fmt.Sprintf("function name cannot be an argument: %s", e.Fun))
+					return nil, nil, nil, false
+				}
+			}
 			args = append(args, es...)
 			argts = append(argts, ts...)
 			stmts = append(stmts, ss...)

internal/shader/syntax_test.go

@@ -3818,3 +3818,19 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #2891
+func TestSyntaxInvalidArgument(t *testing.T) {
+	if _, err := compileToIR([]byte(`package main
+
+func Foo(x int) int {
+	return 0
+}
+
+func Bar() int {
+	return Foo(Foo)
+}
+`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+}