[dev.typeparams] cmd/compile: create .dict Param in the package of the instantiated function

The instantiated functions are created in the source package of the
generic function, so all lookups of symbols should be relative to that
package, so all symbols are consistently in the source package.

Fixes #46575

Change-Id: Iba67b2ba8014a630c5d4e032c0f2f2fbaaedce65
Reviewed-on: https://go-review.googlesource.com/c/go/+/325529
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -558,7 +558,7 @@ func (g *irgen) genericSubst(newsym *types.Sym, nameNode *ir.Name, targs []*type
 	oldt := nameNode.Type()
 	// We also transform a generic method type to the corresponding
 	// instantiated function type where the dictionary is the first parameter.
-	dictionarySym := types.LocalPkg.Lookup(".dict")
+	dictionarySym := newsym.Pkg.Lookup(".dict")
 	dictionaryType := types.Types[types.TUINTPTR]
 	dictionaryName := ir.NewNameAt(gf.Pos(), dictionarySym)
 	typed(dictionaryType, dictionaryName)

test/typeparam/mutualimp.dir/a.go

@@ -0,0 +1,11 @@
+// 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
+
+type X int
+func (x X) M() X { return x }
+
+func F[T interface{ M() U }, U interface{ M() T }]() {}
+func G() { F[X, X]() }

test/typeparam/mutualimp.dir/b.go

@@ -0,0 +1,12 @@
+// 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 b
+
+import "./a"
+
+func H() {
+	a.F[a.X, a.X]()
+	a.G()
+}

test/typeparam/mutualimp.go

@@ -0,0 +1,7 @@
+// compiledir -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