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>

3 files changed, 47 insertions, 0 deletions

diff --git a/test/typeparam/issue50121.dir/a.go b/test/typeparam/issue50121.dir/a.go
new file mode 100644
index 0000000000..9918fa38a6
--- /dev/null
+++ b/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()
+}
diff --git a/test/typeparam/issue50121.dir/main.go b/test/typeparam/issue50121.dir/main.go
new file mode 100644
index 0000000000..71eb44ff62
--- /dev/null
+++ b/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()
+}
diff --git a/test/typeparam/issue50121.go b/test/typeparam/issue50121.go
new file mode 100644
index 0000000000..76930e5e4f
--- /dev/null
+++ b/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