cmd/compile: fix handling of ir.CurFunc during stenciling

The transform functions (specifically transformArgs, which is used from
transformCall/transformReturn) require that ir.CurFunc is set correctly.
Since transformCall() is used on the call of an instantiated generic
function, we need to set ir.CurFunc correctly in stencil(). Also,
correctly save/restore ir.CurFunc in genericSubst().

Without this fix, ir.CurFunc can be nil when we call TransformCall()
from stencil(), which leads to some temp variables being added
incorrectly to ir.TodoFunc (which leads to the fatal panic in the
issue).

Fixes #45722

Change-Id: Iddf4a67d28f2100dde8cde5dbc9ca1e00dad6089
Reviewed-on: https://go-review.googlesource.com/c/go/+/313869
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/test/typeparam/issue45722.go b/test/typeparam/issue45722.go
new file mode 100644
index 0000000000..0d7c20c264
--- /dev/null
+++ b/test/typeparam/issue45722.go
@@ -0,0 +1,34 @@
+// run -gcflags=-G=3
+
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func try[T any](v T, err error) T {
+	if err != nil {
+		panic(err)
+	}
+	return v
+}
+
+func handle(handle func(error)) {
+	if issue := recover(); issue != nil {
+		if e, ok := issue.(error); ok && e != nil {
+			handle(e)
+		} else {
+			handle(fmt.Errorf("%v", e))
+		}
+	}
+}
+
+func main() {
+	defer handle(func(e error) { log.Fatalln(e) })
+	_ = try(fmt.Print(""))
+}