[dev.go2go] cmd/compile/internal/types2: add special case report for generic map key type

Port of go/types CL https://golang.org/cl/247258. (The typeparamsB.go2
version of typeparams.go2 doesn't exist yet for types2.)

Updates #40551.

Change-Id: If2b329372bd4d160b4def8b66cc87f25ed4823fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/248057
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/cmd/compile/internal/types2/testdata/typeparams.go2

@@ -66,7 +66,7 @@ func new(type T)() *T {
 var _ = new /* ERROR cannot use generic function new */
 var _ *int = new(int)()
 
-func _(type T)(map[T /* ERROR invalid map key type */]int) // w/o contract we don't know if T is comparable
+func _(type T)(map[T /* ERROR invalid map key type T \(missing comparable constraint\) */]int) // w/o constraint we don't know if T is comparable
 
 func f1(type T1)(struct{T1}) int
 var _ = f1(int)(struct{T1}{})

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

@@ -550,7 +550,11 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 		// it is safe to continue in any case (was issue 6667).
 		check.atEnd(func() {
 			if !Comparable(typ.key) {
-				check.errorf(e.Key.Pos(), "invalid map key type %s", typ.key)
+				var why string
+				if typ.key.TypeParam() != nil {
+					why = " (missing comparable constraint)"
+				}
+				check.errorf(e.Key.Pos(), "invalid map key type %s%s", typ.key, why)
 			}
 		})