[dev.go2go] go/types: report inferred type when a type argument doesn't match

Fixes #39725.

Change-Id: I48248283ae0eb635d489d8661512f32b98700c1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/240523
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/check_test.go

@@ -126,10 +126,11 @@ var tests = [][]string{
 	// TODO(gri) Eliminate the need to enumerate these tests here.
 	//           Should just traverse that directory.
 	{"fixedbugs/issue39664.go2"},
-	{"fixedbugs/issue39693.go2"},
 	{"fixedbugs/issue39680.go2"},
+	{"fixedbugs/issue39693.go2"},
 	{"fixedbugs/issue39711.go2"},
 	{"fixedbugs/issue39723.go2"},
+	{"fixedbugs/issue39725.go2"},
 	{"fixedbugs/issue39754.go2"},
 	{"fixedbugs/issue39755.go2"},
 	{"fixedbugs/issue39768.go2"},

src/go/types/fixedbugs/issue39725.go2

@@ -0,0 +1,16 @@
+// Copyright 2020 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 f1(type T1, T2)(T1, T2, struct{a T1; b T2})
+func _() {
+	f1(42, string("foo"), struct /* ERROR does not match inferred type struct\{a int; b string\} */ {a, b int}{})
+}
+
+// simplified test case from issue
+func f2(type T)(_ []T, _ func(T))
+func _() {
+	f2([]string{}, func /* ERROR does not match inferred type func\(string\) */ (f []byte) {})
+}

src/go/types/infer.go

@@ -20,13 +20,14 @@ func (check *Checker) infer(pos token.Pos, tparams []*TypeName, params *Tuple, a
 
 	errorf := func(kind string, tpar, targ Type, arg *operand) {
 		// provide a better error message if we can
-		if tpar, _ := tpar.(*TypeParam); tpar != nil {
-			if inferred := u.x.at(tpar.index); inferred != nil {
-				check.errorf(arg.pos(), "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
-				return
-			}
+		targs, _ := u.x.types()
+		smap := makeSubstMap(tparams, targs)
+		inferred := check.subst(arg.pos(), tpar, smap)
+		if inferred != tpar {
+			check.errorf(arg.pos(), "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
+		} else {
+			check.errorf(arg.pos(), "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
 		}
-		check.errorf(arg.pos(), "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
 	}
 
 	// Terminology: generic parameter = function parameter with a type-parameterized type
@@ -96,24 +97,22 @@ func (check *Checker) infer(pos token.Pos, tparams []*TypeName, params *Tuple, a
 
 	// Collect type arguments and check if they all have been determined.
 	// TODO(gri) consider moving this outside this function and then we won't need to pass in pos
-	var targs []Type // lazily allocated
-	for i, tpar := range tparams {
-		targ := u.x.at(i)
-		if targ == nil {
-			ppos := check.fset.Position(tpar.pos).String()
-			check.errorf(pos, "cannot infer %s (%s)", tpar.name, ppos)
-			return nil
-		}
-		if targs == nil {
-			targs = make([]Type, len(tparams))
-		}
-		targs[i] = targ
+	targs, failed := u.x.types()
+	if failed >= 0 {
+		tpar := tparams[failed]
+		ppos := check.fset.Position(tpar.pos).String()
+		check.errorf(pos, "cannot infer %s (%s)", tpar.name, ppos)
+		return nil
 	}
 
 	return targs
 }
 
 // IsParameterized reports whether typ contains any type parameters.
+// TODO(gri) This is not strictly correct. We only want the free
+// type parameters for a given type. (At the moment, the only way
+// to mix free and bound type parameters is through method type parameters
+// on parameterized receiver types - need to investigate.)
 func IsParameterized(typ Type) bool {
 	return isParameterized(typ, make(map[Type]bool))
 }

src/go/types/unify.go

@@ -64,6 +64,23 @@ func (d *typeDesc) at(i int) Type {
 	return nil
 }
 
+// types returns the list of inferred types (via unification) for the type parameters
+// described by d, and an index. If all types were inferred, the returned index is < 0.
+// Otherwise, it is the index of the first type parameter which couldn't be inferred
+// and for which list[index] is nil.
+func (d *typeDesc) types() (list []Type, index int) {
+	list = make([]Type, len(d.tparams))
+	index = -1
+	for i := range d.tparams {
+		t := d.at(i)
+		list[i] = t
+		if index < 0 && t == nil {
+			index = i
+		}
+	}
+	return
+}
+
 // set sets the type typ inferred (via unification) for the i'th type parameter; typ must not be nil.
 // The index i must be a valid type parameter index: 0 <= i < len(d.tparams).
 func (d *typeDesc) set(i int, typ Type) {