[dev.unified] test: add regress test for generic select statements

The Go 1.18 frontend ICEs on select case clauses that involve an
implicit conversion.

Change-Id: I1c0865bf97d8b0a8fbddb0da43333e909df0d38a
Reviewed-on: https://go-review.googlesource.com/c/go/+/414878
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

test/run.go

@@ -1970,6 +1970,7 @@ var go118Failures = setOf(
 	"typeparam/issue53477.go",  // 1.18 compiler mishandles generic interface-interface comparisons from value switch statements
 	"typeparam/mdempsky/16.go", // 1.18 compiler uses interface shape type in failed type assertions
 	"typeparam/mdempsky/17.go", // 1.18 compiler mishandles implicit conversions from range loops
+	"typeparam/mdempsky/18.go", // 1.18 compiler mishandles implicit conversions in select statements
 )
 
 // In all of these cases, the 1.17 compiler reports reasonable errors, but either the

test/typeparam/mdempsky/18.go

@@ -0,0 +1,26 @@
+// run
+
+// Copyright 2022 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.
+
+// Test that implicit conversions to interface type in a select/case
+// clause are compiled correctly.
+
+package main
+
+import "fmt"
+
+func main() { f[int]() }
+
+func f[T any]() {
+	ch := make(chan T)
+	close(ch)
+
+	var i, ok any
+	select {
+	case i, ok = <-ch:
+	}
+
+	fmt.Printf("%T %T\n", i, ok)
+}

test/typeparam/mdempsky/18.out

@@ -0,0 +1 @@
+int bool