cmd/compile: ensure dictionary assignment statements are defining statements

The problem in 51355 is that escape analysis decided that the
dictionary variable was captured by reference instead of by value. We
want dictionaries to always be captured by value.

Escape analysis was confused because it saw what it thought was a
reassignment of the dictionary variable. In fact, it was the only
assignment, it just wasn't marked as the defining assignment. Fix
that.

Add an assert to make sure this stays true.

Fixes #51355

Change-Id: Ifd9342455fa107b113f5ff521a94cdbf1b8a7733
Reviewed-on: https://go-review.googlesource.com/c/go/+/388115
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/escape/escape.go

@@ -10,6 +10,7 @@ import (
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/logopt"
+	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
 )
 
@@ -243,6 +244,9 @@ func (b *batch) flowClosure(k hole, clo *ir.ClosureExpr) {
 		n.SetByval(!loc.addrtaken && !loc.reassigned && n.Type().Size() <= 128)
 		if !n.Byval() {
 			n.SetAddrtaken(true)
+			if n.Sym().Name == typecheck.LocalDictName {
+				base.FatalfAt(n.Pos(), "dictionary variable not captured by value")
+			}
 		}
 
 		if base.Flag.LowerM > 1 {

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

@@ -410,7 +410,8 @@ func (g *genInst) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 	fn, formalParams, formalResults := startClosure(pos, outer, typ)
 
 	// This is the dictionary we want to use.
-	// It may be a constant, or it may be a dictionary acquired from the outer function's dictionary.
+	// It may be a constant, it may be the outer functions's dictionary, or it may be
+	// a subdictionary acquired from the outer function's dictionary.
 	// For the latter, dictVar is a variable in the outer function's scope, set to the subdictionary
 	// read from the outer function's dictionary.
 	var dictVar *ir.Name
@@ -1145,6 +1146,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			newfn.Dcl = append(newfn.Dcl, ldict)
 			as := ir.NewAssignStmt(x.Pos(), ldict, cdict)
 			as.SetTypecheck(1)
+			ldict.Defn = as
 			newfn.Body.Append(as)
 
 			// Create inst info for the instantiated closure. The dict

test/typeparam/issue51355.go

@@ -0,0 +1,31 @@
+// compile -G=3
+
+// 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.
+
+package main
+
+type Cache[E comparable] struct {
+	adder func(...E)
+}
+
+func New[E comparable]() *Cache[E] {
+	c := &Cache[E]{}
+
+	c.adder = func(elements ...E) {
+		for _, value := range elements {
+			value := value
+			go func() {
+				println(value)
+			}()
+		}
+	}
+
+	return c
+}
+
+func main() {
+	c := New[string]()
+	c.adder("test")
+}