[dev.typeparams] cmd/compile: make sure closures inside generic funcs are not compiled

Closures inside generic functions were being added to the g.target.Decls
list during noding, just like other closures. We remove generic
functions/methods from g.target.Decls, so they don't get compiled
(they're only available for export and stenciling). Most closures inside
generic functions/methods were similarly being removed from
g.target.Decls, because they have a generic parameter. But we need to
ensure no closures in generic function/methods are left remaining in
g.target.Decls, since we don't want them transformed and compiled.

So, we set a flag in (*irgen) that records when we are noding a
top-level generic function/method, and don't add any closures to
g.target.Decls when the flag is true.

Updates #47514

Change-Id: Id66b4c41d307ffa8f54cab6ce3646ade81606862
Reviewed-on: https://go-review.googlesource.com/c/go/+/340258
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -102,7 +102,11 @@ func (g *irgen) funcDecl(out *ir.Nodes, decl *syntax.FuncDecl) {
 		g.target.Inits = append(g.target.Inits, fn)
 	}
 
+	if fn.Type().HasTParam() {
+		g.topFuncIsGeneric = true
+	}
 	g.funcBody(fn, decl.Recv, decl.Type, decl.Body)
+	g.topFuncIsGeneric = false
 	if fn.Type().HasTParam() && fn.Body != nil {
 		// Set pointers to the dcls/body of a generic function/method in
 		// the Inl struct, so it is marked for export, is available for

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

@@ -465,8 +465,15 @@ func (g *irgen) funcLit(typ2 types2.Type, expr *syntax.FuncLit) ir.Node {
 		cv.SetWalkdef(1)
 	}
 
+	if g.topFuncIsGeneric {
+		// Don't add any closure inside a generic function/method to the
+		// g.target.Decls list, even though it may not be generic itself.
+		// See issue #47514.
+		return ir.UseClosure(fn.OClosure, nil)
+	} else {
 		return ir.UseClosure(fn.OClosure, g.target)
 	}
+}
 
 func (g *irgen) typeExpr(typ syntax.Expr) *types.Type {
 	n := g.expr(typ)

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

@@ -154,6 +154,11 @@ type irgen struct {
 	// dictionary syms which we need to finish, by writing out any itabconv
 	// entries.
 	dictSymsToFinalize []*delayInfo
+
+	// True when we are compiling a top-level generic function or method. Use to
+	// avoid adding closures of generic functions/methods to the target.Decls
+	// list.
+	topFuncIsGeneric bool
 }
 
 type delayInfo struct {

test/typeparam/issue47514.go

@@ -0,0 +1,20 @@
+// 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.
+
+// Test that closures inside a generic function are not exported,
+// even though not themselves generic.
+
+package main
+
+func Do[T any]() {
+	_ = func() string {
+		return ""
+	}
+}
+
+func main() {
+	Do[int]()
+}