cmd/compile: better error message for invalid untyped operation

For typed vs un-typed operation, the compiler do the conversion
un-conditionally, so if the operation is invalid, the error report is
pointed to the conversion, instead of the invalid operation itself.

To fix this, only do the conversion when the operations are valid
for both types.

Fixes #46749

Change-Id: Ib71c7bcd3ed5454e6df55b6a8db4e0f189259ba7
Reviewed-on: https://go-review.googlesource.com/c/go/+/328050
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue46749.go

@@ -0,0 +1,37 @@
+// errorcheck
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var s string
+var b bool
+var i int
+var iface interface{}
+
+var (
+	_ = "" + b   // ERROR "invalid operation.*mismatched types.*untyped string and bool"
+	_ = "" + i   // ERROR "invalid operation.*mismatched types.*untyped string and int"
+	_ = "" + nil // ERROR "invalid operation.*mismatched types.*untyped string and nil"
+)
+
+var (
+	_ = s + false // ERROR "invalid operation.*mismatched types.*string and untyped bool"
+	_ = s + 1     // ERROR "invalid operation.*mismatched types.*string and untyped int"
+	_ = s + nil   // ERROR "invalid operation.*mismatched types.*string and nil"
+)
+
+var (
+	_ = "" + false // ERROR "invalid operation.*mismatched types.*untyped string and untyped bool"
+	_ = "" + 1     // ERROR "invalid operation.*mismatched types.*untyped string and untyped int"
+)
+
+var (
+	_ = b + 1         // ERROR "invalid operation.*mismatched types.*bool and untyped int"
+	_ = i + false     // ERROR "invalid operation.*mismatched types.*int and untyped bool"
+	_ = iface + 1     // ERROR "invalid operation.*mismatched types.*interface {} and int"
+	_ = iface + 1.0   // ERROR "invalid operation.*mismatched types.*interface {} and float64"
+	_ = iface + false // ERROR "invalid operation.*mismatched types.*interface {} and bool"
+)