go/types: add example test for type inference

This is a port of CL 308973. The only change is to remove a TODO at
inference.go2:100 to improve the error position. The go/types error
position is fine.

Change-Id: Ibf61f3458adde91dec9c7531cbd892ca654a5497
Reviewed-on: https://go-review.googlesource.com/c/go/+/314593
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/examples/inference.go2

@@ -0,0 +1,101 @@
+// Copyright 2021 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.
+
+// This file shows some examples of type inference.
+
+package p
+
+type Ordered interface {
+	type int, float64, string
+}
+
+func min[T Ordered](x, y T) T
+
+func _() {
+	// min can be called with explicit instantiation.
+	_ = min[int](1, 2)
+
+	// Alternatively, the type argument can be inferred from
+	// one of the arguments. Untyped arguments will be considered
+	// last.
+	var x int
+	_ = min(x, x)
+	_ = min(x, 1)
+	_ = min(x, 1.0)
+	_ = min(1, 2)
+	_ = min(1, 2.3 /* ERROR default type float64 .* does not match */ )
+
+	var y float64
+	_ = min(1, y)
+	_ = min(1.2, y)
+	_ = min(1.2, 3.4)
+	_ = min(1.2, 3 /* ERROR default type int .* does not match */ )
+
+	var s string
+	_ = min(s, "foo")
+	_ = min("foo", "bar")
+}
+
+func mixed[T1, T2, T3 any](T1, T2, T3)
+
+func _() {
+	// mixed can be called with explicit instantiation.
+	mixed[int, string, bool](0, "", false)
+
+	// Alternatively, partial type arguments may be provided
+	// (from left to right), and the other may be inferred.
+	mixed[int, string](0, "", false)
+	mixed[int](0, "", false)
+	mixed(0, "", false)
+
+	// Provided type arguments always take precedence over
+	// inferred types.
+	mixed[int, string](1.1 /* ERROR cannot use 1.1 */ , "", false)
+}
+
+func related1[Slice interface{type []Elem}, Elem any](s Slice, e Elem)
+
+func _() {
+	// related1 can be called with explicit instantiation.
+	var si []int
+	related1[[]int, int](si, 0)
+
+	// Alternatively, the 2nd type argument can be inferred
+	// from the first one through constraint type inference.
+	var ss []string
+	_ = related1[[]string]
+	related1[[]string](ss, "foo")
+
+	// A type argument inferred from another explicitly provided
+	// type argument overrides whatever value argument type is given.
+	related1[[]string](ss, 0 /* ERROR cannot use 0 */ )
+
+	// A type argument may be inferred from a value argument
+	// and then help infer another type argument via constraint
+	// type inference.
+	related1(si, 0)
+	related1(si, "foo" /* ERROR cannot use "foo" */ )
+}
+
+func related2[Elem any, Slice interface{type []Elem}](e Elem, s Slice)
+
+func _() {
+	// related2 can be called with explicit instantiation.
+	var si []int
+	related2[int, []int](0, si)
+
+	// Alternatively, the 2nd type argument can be inferred
+	// from the first one through constraint type inference.
+	var ss []string
+	_ = related2[string]
+	related2[string]("foo", ss)
+
+	// A type argument may be inferred from a value argument
+	// and then help infer another type argument via constraint
+	// type inference. Untyped arguments are always considered
+	// last.
+	related2(1.2, []float64{})
+	related2(1.0, []int{})
+	related2 /* ERROR does not satisfy */ (float64(1.0), []int{})
+}