cmd/compile: enable more lenient type inference for untyped arguments

This enables the implementation for proposal #58671, which is
a likely accept. By enabling it early we get a bit extra soak
time for this feature. The change can be reverted trivially, if
need be.

For #58671.

Change-Id: Id6c27515e45ff79f4f1d2fc1706f3f672ccdd1ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/495955
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -50,8 +50,9 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 			}
 			base.ErrorfAt(m.makeXPos(terr.Pos), terr.Code, "%s", msg)
 		},
-		Importer: &importer,
+		Importer:            &importer,
-		Sizes:    &gcSizes{},
+		Sizes:               &gcSizes{},
+		InferMaxDefaultType: true, // #58671
 	}
 	info := &types2.Info{
 		StoreTypesInSyntax: true,

src/cmd/compile/internal/types2/stdlib_test.go

@@ -139,8 +139,9 @@ func testTestDir(t *testing.T, path string, ignore ...string) {
 		file, err := syntax.ParseFile(filename, nil, nil, 0)
 		if err == nil {
 			conf := Config{
-				GoVersion: goVersion,
+				GoVersion:           goVersion,
-				Importer:  stdLibImporter,
+				Importer:            stdLibImporter,
+				InferMaxDefaultType: true,
 			}
 			_, err = conf.Check(filename, []*syntax.File{file}, nil)
 		}

src/go/types/stdlib_test.go

@@ -143,6 +143,7 @@ func testTestDir(t *testing.T, path string, ignore ...string) {
 				GoVersion: goVersion,
 				Importer:  stdLibImporter,
 			}
+			*boolFieldAddr(&conf, "_InferMaxDefaultType") = true
 			_, err = conf.Check(filename, fset, []*ast.File{file}, nil)
 		}
 

test/fixedbugs/issue58671.go

@@ -0,0 +1,19 @@
+// run
+
+// Copyright 2023 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
+
+func g[P any](...P) P { var zero P; return zero }
+
+var (
+	_ int        = g(1, 2)
+	_ rune       = g(1, 'a')
+	_ float64    = g(1, 'a', 2.3)
+	_ float64    = g('a', 2.3)
+	_ complex128 = g(2.3, 'a', 1i)
+)
+
+func main() {}