go/types, types2: better error messages for certain type mismatches

When an untyped operand of a (typically binary) operation does not
match the type of the operand and an implicit conversion is not
possible, the error message should report a "type mismatch".

The type-checkers mostly did so, but not for untyped numeric types
to other types (e.g. an untyped int vs a function); in those cases
it reported that the (impossible) conversion failed.

Fix this for numeric types.
This also improves the position and messages for some incorrect
min/max built-in calls.

Fixes #73428.

Change-Id: I8af071918b73fcc72f16cc61858d7baca57fc259
Reviewed-on: https://go-review.googlesource.com/c/go/+/682495
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <mark@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/types2/expr.go

@@ -895,6 +895,10 @@ func (check *Checker) matchTypes(x, y *operand) {
 		if isTyped(x.typ) && isTyped(y.typ) {
 			return false
 		}
+		// A numeric type can only convert to another numeric type.
+		if allNumeric(x.typ) != allNumeric(y.typ) {
+			return false
+		}
 		// An untyped operand may convert to its default type when paired with an empty interface
 		// TODO(gri) This should only matter for comparisons (the only binary operation that is
 		//           valid with interfaces), but in that case the assignability check should take

src/go/types/expr.go

@@ -887,6 +887,10 @@ func (check *Checker) matchTypes(x, y *operand) {
 		if isTyped(x.typ) && isTyped(y.typ) {
 			return false
 		}
+		// A numeric type can only convert to another numeric type.
+		if allNumeric(x.typ) != allNumeric(y.typ) {
+			return false
+		}
 		// An untyped operand may convert to its default type when paired with an empty interface
 		// TODO(gri) This should only matter for comparisons (the only binary operation that is
 		//           valid with interfaces), but in that case the assignability check should take

src/internal/types/errors/codes.go

@@ -881,7 +881,9 @@ const (
 	// context in which it is used.
 	//
 	// Example:
-	//  var _ = 1 + []int{}
+	//  func f[T ~int8 | ~int16 | ~int32 | ~int64](x T) T {
+	//  	return x + 1024
+	//  }
 	InvalidUntypedConversion
 
 	// BadOffsetofSyntax occurs when unsafe.Offsetof is called with an argument

src/internal/types/testdata/check/builtins1.go

@@ -211,7 +211,7 @@ func _[
 	var x2 P2
 	_ = max(x2)
 	_ = max(x2, x2)
-	_ = max(1, 2 /* ERROR "cannot convert 2 (untyped int constant) to type P2" */, x2) // error at 2 because max is 2
+	_ = max(1, 2, x2 /* ERROR "mismatched types untyped int (previous argument) and P2 (type of x2)" */ )
 
 	_ = max(x1, x2 /* ERROR "mismatched types P1 (previous argument) and P2 (type of x2)" */ )
 }
@@ -232,7 +232,7 @@ func _[
 	var x2 P2
 	_ = min(x2)
 	_ = min(x2, x2)
-	_ = min(1 /* ERROR "cannot convert 1 (untyped int constant) to type P2" */ , 2, x2) // error at 1 because min is 1
+	_ = min(1, 2, x2 /* ERROR "mismatched types untyped int (previous argument) and P2 (type of x2)" */ )
 
 	_ = min(x1, x2 /* ERROR "mismatched types P1 (previous argument) and P2 (type of x2)" */ )
 }

src/internal/types/testdata/check/expr2.go

@@ -201,7 +201,7 @@ func interfaces() {
 	var s11 S11
 	var s2 S2
 
-	_ = i == 0 /* ERROR "cannot convert" */
+	_ = i == 0 /* ERROR "invalid operation: i == 0 (mismatched types interface{m() int} and untyped int)" */
 	_ = i == s1 /* ERROR "mismatched types" */
 	_ = i == &s1
 	_ = i == &s11

src/internal/types/testdata/fixedbugs/issue60434.go

@@ -13,5 +13,5 @@ var s struct{ x int }
 
 func _() {
 	f(s.y /* ERROR "s.y undefined" */)
-	f(1 /* ERROR "cannot convert 1" */ / s)
+	f(1 /* ERROR "invalid operation: 1 / s (mismatched types untyped int and struct{x int})" */ / s)
 }

src/internal/types/testdata/fixedbugs/issue73428.go

@@ -0,0 +1,13 @@
+// Copyright 2025 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
+
+func f() {}
+
+const c = 0
+
+var v int
+var _ = f < c // ERROR "invalid operation: f < c (mismatched types func() and untyped int)"
+var _ = f < v // ERROR "invalid operation: f < v (mismatched types func() and int)"

test/fixedbugs/issue46749.go

@@ -31,7 +31,7 @@ var (
 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 + 1     // ERROR "invalid operation.*mismatched types.*interface *{} and untyped int"
+	_ = iface + 1.0   // ERROR "invalid operation.*mismatched types.*interface *{} and untyped float"
 	_ = iface + false // ERROR "invalid operation.*mismatched types.*interface *{} and bool"
 )