go/types, types2: fix parameter order dependence in type inference

If we have more than two function arguments to a generic function,
we may have arguments with named and unnamed types. If that is the
case, permutate params and args such that the arguments with named
types are first in the list. This way, independent of parameter
ordering, the type inference will produce the same result.

This extra step is not explicitly outlined in the spec yet but we
all agree that (parameter) order independence is an invariant that
we should uphold for type inference. As we move towards less
operational and more descriptive rules for type inference, we will
incorporate this property as well.

The actual fix for this bug existed before 1.18 but was not enabled.
This CL merely enables the fix (switches a flag) and adjusts some
tests.

Fixes #43056.

Change-Id: Ie4e40cf8438dfd82fa94b78068e4f6f6f53f83e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/413459
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

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

@@ -128,11 +128,8 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 	// named and unnamed types are passed to parameters with identical type, different types
 	// (named vs underlying) may be inferred depending on the order of the arguments.
 	// By ensuring that named types are seen first, order dependence is avoided and unification
-	// succeeds where it can.
-	//
-	// This code is disabled for now pending decision whether we want to address cases like
-	// these and make the spec on type inference more complicated (see issue #43056).
-	const enableArgSorting = false
+	// succeeds where it can (issue #43056).
+	const enableArgSorting = true
 	if m := len(args); m >= 2 && enableArgSorting {
 		// Determine indices of arguments with named and unnamed types.
 		var named, unnamed []int

src/cmd/compile/internal/types2/testdata/examples/functions.go

@@ -182,7 +182,7 @@ func _() {
 	type myString string
 	var s1 string
 	g3(nil, "1", myString("2"), "3")
-	g3(&s1, "1", myString /* ERROR does not match */ ("2"), "3")
+	g3(& /* ERROR does not match */ s1, "1", myString("2"), "3")
 	_ = s1
 
 	type myStruct struct{x int}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue43056.go

@@ -14,7 +14,7 @@ func _() {
 	var j func(F)
 
 	f(i, j)
-	// f(j, i) // disabled for now
+	f(j, i)
 }
 
 // example from issue
@@ -27,5 +27,5 @@ func _() {
 	var j interface{ Equal(I) bool }
 
 	g(i, j)
-	// g(j, i) // disabled for now
+	g(j, i)
 }

src/go/types/infer.go

@@ -128,11 +128,8 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 	// named and unnamed types are passed to parameters with identical type, different types
 	// (named vs underlying) may be inferred depending on the order of the arguments.
 	// By ensuring that named types are seen first, order dependence is avoided and unification
-	// succeeds where it can.
-	//
-	// This code is disabled for now pending decision whether we want to address cases like
-	// these and make the spec on type inference more complicated (see issue #43056).
-	const enableArgSorting = false
+	// succeeds where it can (issue #43056).
+	const enableArgSorting = true
 	if m := len(args); m >= 2 && enableArgSorting {
 		// Determine indices of arguments with named and unnamed types.
 		var named, unnamed []int

src/go/types/testdata/examples/functions.go

@@ -182,7 +182,7 @@ func _() {
 	type myString string
 	var s1 string
 	g3(nil, "1", myString("2"), "3")
-	g3(&s1, "1", myString /* ERROR does not match */ ("2"), "3")
+	g3(& /* ERROR does not match */ s1, "1", myString("2"), "3")
 
 	type myStruct struct{x int}
 	var s2 myStruct

src/go/types/testdata/fixedbugs/issue43056.go

@@ -14,7 +14,7 @@ func _() {
 	var j func(F)
 
 	f(i, j)
-	// f(j, i) // disabled for now
+	f(j, i)
 }
 
 // example from issue
@@ -27,5 +27,5 @@ func _() {
 	var j interface{ Equal(I) bool }
 
 	g(i, j)
-	// g(j, i) // disabled for now
+	g(j, i)
 }