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>

src/cmd/compile/internal/noder/stencil.go

@@ -52,6 +52,8 @@ func (g *irgen) stencil() {
 				// Skip any generic functions
 				continue
 			}
+			// transformCall() below depends on CurFunc being set.
+			ir.CurFunc = decl.(*ir.Func)
 
 		case ir.OAS, ir.OAS2, ir.OAS2DOTTYPE, ir.OAS2FUNC, ir.OAS2MAPR, ir.OAS2RECV, ir.OASOP:
 			// These are all the various kinds of global assignments,
@@ -127,6 +129,7 @@ func (g *irgen) stencil() {
 		if base.Flag.W > 1 && modified {
 			ir.Dump(fmt.Sprintf("\nmodified %v", decl), decl)
 		}
+		ir.CurFunc = nil
 		// We may have seen new fully-instantiated generic types while
 		// instantiating any needed functions/methods in the above
 		// function. If so, instantiate all the methods of those types
@@ -275,6 +278,9 @@ func (g *irgen) genericSubst(newsym *types.Sym, nameNode *ir.Name, targs []ir.No
 	newf.Nname.Func = newf
 	newf.Nname.Defn = newf
 	newsym.Def = newf.Nname
+	savef := ir.CurFunc
+	// transformCall/transformReturn (called during stenciling of the body)
+	// depend on ir.CurFunc being set.
 	ir.CurFunc = newf
 
 	assert(len(tparams) == len(targs))
@@ -310,7 +316,7 @@ func (g *irgen) genericSubst(newsym *types.Sym, nameNode *ir.Name, targs []ir.No
 
 	// Make sure name/type of newf is set before substituting the body.
 	newf.Body = subst.list(gf.Body)
-	ir.CurFunc = nil
+	ir.CurFunc = savef
 
 	return newf
 }

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(""))
+}