cmd/compile: fix bad order of evaluation for multi-value f()(g()) calls

The compiler use to compile f()(g()) as:

	t1, t2 := g()
	f()(t1, t2)

That violates the Go spec, since when "..., all function calls, ... are
evaluated in lexical left-to-right order"

This PR fixes the bug by compiling f()(g()) as:

	t0 := f()
	t1, t2 := g()
	t0(t1, t2)

to make "f()" to be evaluated before "g()".

Fixes #50672

Change-Id: I6a766f3dfc7347d10f8fa3a151f6a5ea79bcf818
Reviewed-on: https://go-review.googlesource.com/c/go/+/392834
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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

@@ -162,6 +162,7 @@ func transformCall(n *ir.CallExpr) {
 	ir.SetPos(n)
 	// n.Type() can be nil for calls with no return value
 	assert(n.Typecheck() == 1)
+	typecheck.RewriteNonNameCall(n)
 	transformArgs(n)
 	l := n.X
 	t := l.Type()

src/cmd/compile/internal/typecheck/const.go

@@ -734,35 +734,40 @@ func IndexConst(n ir.Node) int64 {
 	return ir.IntVal(types.Types[types.TINT], v)
 }
 
+// callOrChan reports whether n is a call or channel operation.
+func callOrChan(n ir.Node) bool {
+	switch n.Op() {
+	case ir.OAPPEND,
+		ir.OCALL,
+		ir.OCALLFUNC,
+		ir.OCALLINTER,
+		ir.OCALLMETH,
+		ir.OCAP,
+		ir.OCLOSE,
+		ir.OCOMPLEX,
+		ir.OCOPY,
+		ir.ODELETE,
+		ir.OIMAG,
+		ir.OLEN,
+		ir.OMAKE,
+		ir.ONEW,
+		ir.OPANIC,
+		ir.OPRINT,
+		ir.OPRINTN,
+		ir.OREAL,
+		ir.ORECOVER,
+		ir.ORECV,
+		ir.OUNSAFEADD,
+		ir.OUNSAFESLICE:
+		return true
+	}
+	return false
+}
+
 // anyCallOrChan reports whether n contains any calls or channel operations.
 func anyCallOrChan(n ir.Node) bool {
 	return ir.Any(n, func(n ir.Node) bool {
-		switch n.Op() {
-		case ir.OAPPEND,
-			ir.OCALL,
-			ir.OCALLFUNC,
-			ir.OCALLINTER,
-			ir.OCALLMETH,
-			ir.OCAP,
-			ir.OCLOSE,
-			ir.OCOMPLEX,
-			ir.OCOPY,
-			ir.ODELETE,
-			ir.OIMAG,
-			ir.OLEN,
-			ir.OMAKE,
-			ir.ONEW,
-			ir.OPANIC,
-			ir.OPRINT,
-			ir.OPRINTN,
-			ir.OREAL,
-			ir.ORECOVER,
-			ir.ORECV,
-			ir.OUNSAFEADD,
-			ir.OUNSAFESLICE:
-			return true
-		}
-		return false
+		return callOrChan(n)
 	})
 }
 

src/cmd/compile/internal/typecheck/func.go

@@ -343,6 +343,7 @@ func tcCall(n *ir.CallExpr, top int) ir.Node {
 		return tcConv(n)
 	}
 
+	RewriteNonNameCall(n)
 	typecheckargs(n)
 	t := l.Type()
 	if t == nil {

src/cmd/compile/internal/typecheck/typecheck.go

@@ -867,6 +867,42 @@ func typecheckargs(n ir.InitNode) {
 	RewriteMultiValueCall(n, list[0])
 }
 
+// RewriteNonNameCall replaces non-Name call expressions with temps,
+// rewriting f()(...) to t0 := f(); t0(...).
+func RewriteNonNameCall(n *ir.CallExpr) {
+	np := &n.X
+	if inst, ok := (*np).(*ir.InstExpr); ok && inst.Op() == ir.OFUNCINST {
+		np = &inst.X
+	}
+	if dot, ok := (*np).(*ir.SelectorExpr); ok && (dot.Op() == ir.ODOTMETH || dot.Op() == ir.ODOTINTER || dot.Op() == ir.OMETHVALUE) {
+		np = &dot.X // peel away method selector
+	}
+
+	// Check for side effects in the callee expression.
+	// We explicitly special case new(T) though, because it doesn't have
+	// observable side effects, and keeping it in place allows better escape analysis.
+	if !ir.Any(*np, func(n ir.Node) bool { return n.Op() != ir.ONEW && callOrChan(n) }) {
+		return
+	}
+
+	// See comment (1) in RewriteMultiValueCall.
+	static := ir.CurFunc == nil
+	if static {
+		ir.CurFunc = InitTodoFunc
+	}
+
+	tmp := Temp((*np).Type())
+	as := ir.NewAssignStmt(base.Pos, tmp, *np)
+	as.Def = true
+	*np = tmp
+
+	if static {
+		ir.CurFunc = nil
+	}
+
+	n.PtrInit().Append(Stmt(as))
+}
+
 // RewriteMultiValueCall rewrites multi-valued f() to use temporaries,
 // so the backend wouldn't need to worry about tuple-valued expressions.
 func RewriteMultiValueCall(n ir.InitNode, call ir.Node) {
@@ -874,7 +910,7 @@ func RewriteMultiValueCall(n ir.InitNode, call ir.Node) {
 	// be executed during the generated init function. However,
 	// init.go hasn't yet created it. Instead, associate the
 	// temporary variables with  InitTodoFunc for now, and init.go
-	// will reassociate them later when it's appropriate.
+	// will reassociate them later when it's appropriate. (1)
 	static := ir.CurFunc == nil
 	if static {
 		ir.CurFunc = InitTodoFunc

test/fixedbugs/issue50672.go

@@ -0,0 +1,105 @@
+// 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.
+
+package main
+
+var ok = false
+
+func f() func(int, int) int {
+	ok = true
+	return func(int, int) int { return 0 }
+}
+
+func g() (int, int) {
+	if !ok {
+		panic("FAIL")
+	}
+	return 0, 0
+}
+
+var _ = f()(g())
+
+func main() {
+	f1()
+	f2()
+	f3()
+	f4()
+}
+
+func f1() {
+	ok := false
+
+	f := func() func(int, int) {
+		ok = true
+		return func(int, int) {}
+	}
+	g := func() (int, int) {
+		if !ok {
+			panic("FAIL")
+		}
+		return 0, 0
+	}
+
+	f()(g())
+}
+
+type S struct{}
+
+func (S) f(int, int) {}
+
+func f2() {
+	ok := false
+
+	f := func() S {
+		ok = true
+		return S{}
+	}
+	g := func() (int, int) {
+		if !ok {
+			panic("FAIL")
+		}
+		return 0, 0
+	}
+
+	f().f(g())
+}
+
+func f3() {
+	ok := false
+
+	f := func() []func(int, int) {
+		ok = true
+		return []func(int, int){func(int, int) {}}
+	}
+	g := func() (int, int) {
+		if !ok {
+			panic("FAIL")
+		}
+		return 0, 0
+	}
+	f()[0](g())
+}
+
+type G[T any] struct{}
+
+func (G[T]) f(int, int) {}
+
+func f4() {
+	ok := false
+
+	f := func() G[int] {
+		ok = true
+		return G[int]{}
+	}
+	g := func() (int, int) {
+		if !ok {
+			panic("FAIL")
+		}
+		return 0, 0
+	}
+
+	f().f(g())
+}