go/types, types2: use correct parameter list when checking argument passing

The existing code was simply wrong: we cannot ever use the result
signature parameter list (rsig.params) if sigParams was adjusted
for variadic functions. If it was adjusted, we always must either
use sigParams or its separately instantiated version.

In the condition "n > 0 && adjusted", the "n > 0" should have
been in either of the respective "if statement" branches.

Simplified the code by merging with the result signature parameter
update.

Fixes #61931.

Change-Id: I5d39bc8bbc4dd85c7c985055d29532b4b176955e
Reviewed-on: https://go-review.googlesource.com/c/go/+/519456
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -610,21 +610,18 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 			return // error already reported
 		}
 
-		// compute result signature: instantiate if needed
+		// update result signature: instantiate if needed
-		rsig = sig
 		if n > 0 {
 			rsig = check.instantiateSignature(call.Pos(), call.Fun, sig, targs[:n], xlist)
-		}
+			// If the callee's parameter list was adjusted we need to update (instantiate)
-
+			// it separately. Otherwise we can simply use the result signature's parameter
-		// Optimization: Only if the callee's parameter list was adjusted do we need to
+			// list.
-		// compute it from the adjusted list; otherwise we can simply use the result
+			if adjusted {
-		// signature's parameter list. We only need the n type parameters and arguments
-		// of the callee.
-		if n > 0 && adjusted {
 				sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(tparams[:n], targs[:n]), nil, check.context()).(*Tuple)
 			} else {
 				sigParams = rsig.params
 			}
+		}
 
 		// compute argument signatures: instantiate if needed
 		j := n

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

@@ -900,3 +900,23 @@ func _cgoCheckResult(interface{})
 		*boolFieldAddr(cfg, "go115UsesCgo") = true
 	})
 }
+
+func TestIssue61931(t *testing.T) {
+	const src = `
+package p
+
+func A(func(any), ...any) {}
+func B[T any](T)          {}
+
+func _() {
+	A(B, nil // syntax error: missing ',' before newline in argument list
+}
+`
+	f, err := syntax.Parse(syntax.NewFileBase(pkgName(src)), strings.NewReader(src), func(error) {}, nil, 0)
+	if err == nil {
+		t.Fatal("expected syntax error")
+	}
+
+	var conf Config
+	conf.Check(f.PkgName.Value, []*syntax.File{f}, nil) // must not panic
+}

src/go/types/call.go

@@ -612,21 +612,18 @@ func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type
 			return // error already reported
 		}
 
-		// compute result signature: instantiate if needed
+		// update result signature: instantiate if needed
-		rsig = sig
 		if n > 0 {
 			rsig = check.instantiateSignature(call.Pos(), call.Fun, sig, targs[:n], xlist)
-		}
+			// If the callee's parameter list was adjusted we need to update (instantiate)
-
+			// it separately. Otherwise we can simply use the result signature's parameter
-		// Optimization: Only if the callee's parameter list was adjusted do we need to
+			// list.
-		// compute it from the adjusted list; otherwise we can simply use the result
+			if adjusted {
-		// signature's parameter list. We only need the n type parameters and arguments
-		// of the callee.
-		if n > 0 && adjusted {
 				sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(tparams[:n], targs[:n]), nil, check.context()).(*Tuple)
 			} else {
 				sigParams = rsig.params
 			}
+		}
 
 		// compute argument signatures: instantiate if needed
 		j := n

src/go/types/issues_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/importer"
+	"go/parser"
 	"go/token"
 	"internal/testenv"
 	"regexp"
@@ -908,3 +909,24 @@ func _cgoCheckResult(interface{})
 		*boolFieldAddr(cfg, "go115UsesCgo") = true
 	})
 }
+
+func TestIssue61931(t *testing.T) {
+	const src = `
+package p
+
+func A(func(any), ...any) {}
+func B[T any](T)          {}
+
+func _() {
+	A(B, nil // syntax error: missing ',' before newline in argument list
+}
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, pkgName(src), src, 0)
+	if err == nil {
+		t.Fatal("expected syntax error")
+	}
+
+	var conf Config
+	conf.Check(f.Name.Name, fset, []*ast.File{f}, nil) // must not panic
+}