[dev.go2go] go/types: add special case report for generic map key type

Fixes #40551.

Change-Id: Ifc10dbdcdbee6af5bdbfc36ec78e1a4361672acc
GitHub-Last-Rev: 8d373de
GitHub-Pull-Request: #40628
Reviewed-on: https://go-review.googlesource.com/c/go/+/247258
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: tdakkota

src/go/types/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 constraint 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/go/types/testdata/typeparamsB.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 constraint 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/go/types/typexpr.go

@@ -528,7 +528,11 @@ func (check *Checker) typInternal(e0 ast.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)
 			}
 		})