cmd/compile: fix bug with Inferred targs

We were using the type from the wrong Node (the partially filled-in
FUNCINST) rather than the original function node - which is pointed to
by the OFUNCINST)) to set the final fully-substituted type of the
OFUNCINST. So fixed the node reference. Also, added check so we don't do
any work at all if the OFUNCINST already has all type args filled in.

Added few extra cases to the test file issue48030.go, to cover
fully-specified type args, partially inferred type args, and fully
inferred type args.

Fixes #48030

Change-Id: If9e4f2e0514d68b9d241f30c423259133932b25b
Reviewed-on: https://go-review.googlesource.com/c/go/+/346229
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -126,13 +126,15 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 				targs[i] = ir.TypeNode(g.typ(inferred.TArgs.At(i)))
 			}
 			if fun.Op() == ir.OFUNCINST {
-				// Replace explicit type args with the full list that
-				// includes the additional inferred type args.
-				// Substitute the type args for the type params in
-				// the generic function's type.
-				fun.(*ir.InstExpr).Targs = targs
-				newt := g.substType(fun.Type(), fun.Type().TParams(), targs)
-				typed(newt, fun)
+				if len(fun.(*ir.InstExpr).Targs) < len(targs) {
+					// Replace explicit type args with the full list that
+					// includes the additional inferred type args.
+					// Substitute the type args for the type params in
+					// the generic function's type.
+					fun.(*ir.InstExpr).Targs = targs
+					newt := g.substType(fun.(*ir.InstExpr).X.Type(), fun.(*ir.InstExpr).X.Type().TParams(), targs)
+					typed(newt, fun)
+				}
 			} else {
 				// Create a function instantiation here, given there
 				// are only inferred type args (e.g. min(5,6), where

test/typeparam/issue48030.go

@@ -0,0 +1,26 @@
+// 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
+
+type Src[T any] func() Src[T]
+
+func Seq[T any]() Src[T] {
+	return nil
+}
+
+func Seq2[T1 any, T2 any](v1 T1, v2 T2) Src[T2] {
+	return nil
+}
+
+func main() {
+	// Type args fully supplied
+	Seq[int]()
+	// Partial inference of type args
+	Seq2[int](5, "abc")
+	// Full inference of type args
+	Seq2(5, "abc")
+}