[dev.typeparams] cmd/compile: handle ++/-- in noder2 for operands with generic type

types2 will have already proved the expression's type is compatible, so
just assign the one const to have the same type as the operand.

Fixes #47258.

Change-Id: If0844e6bf6d0a5e6b11453b87df71353863ccc5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/336009
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/helpers.go

@@ -337,5 +337,15 @@ var one = constant.MakeInt64(1)
 
 func IncDec(pos src.XPos, op ir.Op, x ir.Node) *ir.AssignOpStmt {
 	assert(x.Type() != nil)
-	return ir.NewAssignOpStmt(pos, op, x, typecheck.DefaultLit(ir.NewBasicLit(pos, one), x.Type()))
+	bl := ir.NewBasicLit(pos, one)
+	if x.Type().HasTParam() {
+		// If the operand is generic, then types2 will have proved it must be
+		// a type that fits with increment/decrement, so just set the type of
+		// "one" to n.Type(). This works even for types that are eventually
+		// float or complex.
+		typed(x.Type(), bl)
+	} else {
+		bl = typecheck.DefaultLit(bl, x.Type())
+	}
+	return ir.NewAssignOpStmt(pos, op, x, bl)
 }

test/typeparam/issue47258.go

@@ -0,0 +1,32 @@
+// 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"
+)
+
+type Numeric interface {
+	int32|int64|float64|complex64
+}
+
+//go:noline
+func inc[T Numeric](x T) T {
+	x++
+	return x
+}
+func main() {
+	if got, want := inc(int32(5)), int32(6); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := inc(float64(5)), float64(6.0); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := inc(complex64(5)), complex64(6.0); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+}