cmd/compile: avoid re-instantiating method that is already imported

We can import an shape-instantiated function/method for inlining
purposes. If we are instantiating the methods of a instantiated type
that we have seen, and it happens to need a shape instantiation that we
have imported, then don't re-create the instantiation, since we will end
up with conflicting/duplicate definitions for the instantiation symbol.
Instead, we can just use the existing imported instantation, and enter
it in the instInfoMap[].

Fixes #50121

Change-Id: I6eeb8786faad71106e261e113048b579afad04fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/371414
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -684,7 +684,22 @@ func (g *genInst) getInstantiation(nameNode *ir.Name, shapes []*types.Type, isMe
 		}
 		info.dictInfo.shapeToBound = make(map[*types.Type]*types.Type)
 
-		// genericSubst fills in info.dictParam and info.tparamToBound.
+		if sym.Def != nil {
+			// This instantiation must have been imported from another
+			// package (because it was needed for inlining), so we should
+			// not re-generate it and have conflicting definitions for the
+			// symbol (issue #50121). It will have already gone through the
+			// dictionary transformations of dictPass, so we don't actually
+			// need the info.dictParam and info.shapeToBound info filled in
+			// below. We just set the imported instantiation as info.fun.
+			assert(sym.Pkg != types.LocalPkg)
+			info.fun = sym.Def.(*ir.Name).Func
+			assert(info.fun != nil)
+			g.instInfoMap[sym] = info
+			return info
+		}
+
+		// genericSubst fills in info.dictParam and info.shapeToBound.
 		st := g.genericSubst(sym, nameNode, shapes, isMeth, info)
 		info.fun = st
 		g.instInfoMap[sym] = info
@@ -722,7 +737,7 @@ type subster struct {
 // args shapes. For a method with a generic receiver, it returns an instantiated
 // function type where the receiver becomes the first parameter. For either a generic
 // method or function, a dictionary parameter is the added as the very first
-// parameter. genericSubst fills in info.dictParam and info.tparamToBound.
+// parameter. genericSubst fills in info.dictParam and info.shapeToBound.
 func (g *genInst) genericSubst(newsym *types.Sym, nameNode *ir.Name, shapes []*types.Type, isMethod bool, info *instInfo) *ir.Func {
 	var tparams []*types.Type
 	if isMethod {

test/typeparam/issue50121.dir/a.go

@@ -0,0 +1,22 @@
+// 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 a
+
+import (
+	"constraints"
+	"math/rand"
+)
+
+type Builder[T constraints.Integer] struct{}
+
+func (r Builder[T]) New() T {
+	return T(rand.Int())
+}
+
+var IntBuilder = Builder[int]{}
+
+func BuildInt() int {
+	return IntBuilder.New()
+}

test/typeparam/issue50121.dir/main.go

@@ -0,0 +1,18 @@
+// 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 (
+	"a"
+)
+
+//go:noinline
+func BuildInt() int {
+	return a.BuildInt()
+}
+
+func main() {
+	BuildInt()
+}

test/typeparam/issue50121.go

@@ -0,0 +1,7 @@
+// rundir -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 ignored