go/types, types2: better internal comment, added suitable test case

Change-Id: If55cd001ab3d274cd9c61c06f73bb98162aa12a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/471019
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -73,8 +73,25 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 
 	// Substitute type arguments for their respective type parameters in params,
 	// if any. Note that nil targs entries are ignored by check.subst.
-	// TODO(gri) Can we avoid this (we're setting known type arguments below,
+	// We do this for better error messages; it's not needed for correctness.
-	//           but that doesn't impact the isParameterized check for now).
+	// For instance, given:
+	//
+	//   func f[P, Q any](P, Q) {}
+	//
+	//   func _(s string) {
+	//           f[int](s, s) // ERROR
+	//   }
+	//
+	// With substitution, we get the error:
+	//   "cannot use s (variable of type string) as int value in argument to f[int]"
+	//
+	// Without substitution we get the (worse) error:
+	//   "type string of s does not match inferred type int for P"
+	// even though the type int was provided (not inferred) for P.
+	//
+	// TODO(gri) We might be able to finesse this in the error message reporting
+	//           (which only happens in case of an error) and then avoid doing
+	//           the substitution (which always happens).
 	if params.Len() > 0 {
 		smap := makeSubstMap(tparams, targs)
 		params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple)

src/go/types/infer.go

@@ -75,8 +75,25 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 
 	// Substitute type arguments for their respective type parameters in params,
 	// if any. Note that nil targs entries are ignored by check.subst.
-	// TODO(gri) Can we avoid this (we're setting known type arguments below,
+	// We do this for better error messages; it's not needed for correctness.
-	//           but that doesn't impact the isParameterized check for now).
+	// For instance, given:
+	//
+	//   func f[P, Q any](P, Q) {}
+	//
+	//   func _(s string) {
+	//           f[int](s, s) // ERROR
+	//   }
+	//
+	// With substitution, we get the error:
+	//   "cannot use s (variable of type string) as int value in argument to f[int]"
+	//
+	// Without substitution we get the (worse) error:
+	//   "type string of s does not match inferred type int for P"
+	// even though the type int was provided (not inferred) for P.
+	//
+	// TODO(gri) We might be able to finesse this in the error message reporting
+	//           (which only happens in case of an error) and then avoid doing
+	//           the substitution (which always happens).
 	if params.Len() > 0 {
 		smap := makeSubstMap(tparams, targs)
 		params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple)

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

@@ -102,3 +102,11 @@ func (p *Settable) Set(s string) {
 }
 
 var _ = FromStrings[Settable]([]string{"1", "2"})
+
+// Suitable error message when the type parameter is provided (rather than inferred).
+
+func f8[P, Q any](P, Q) {}
+
+func _(s string) {
+	f8[int](s /* ERROR "cannot use s (variable of type string) as int value in argument to f8[int]" */ , s)
+}