cmd/compile/internal/syntax: make valid type parameter list in presence of errors

Make sure the parser fills in names and types for type parameter
lists, even in the case of errors.

While at it, adjust some of the test functions to accept generic
code and report all syntax errors.

Added offending source as test for types2.

Fixes #47996.

Change-Id: I449bcf5e2cb80fa2a24cdd3945f484bfca218a06
Reviewed-on: https://go-review.googlesource.com/c/go/+/345476
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

4 files changed, 11 insertions, 14 deletions

diff --git a/src/cmd/compile/internal/syntax/dumper_test.go b/src/cmd/compile/internal/syntax/dumper_test.go
index 22680dce78..033283a352 100644
--- a/src/cmd/compile/internal/syntax/dumper_test.go
+++ b/src/cmd/compile/internal/syntax/dumper_test.go
@@ -13,11 +13,7 @@ func TestDump(t *testing.T) {
 		t.Skip("skipping test in short mode")
 	}
 
-	// provide a no-op error handler so parsing doesn't stop after first error
-	ast, err := ParseFile(*src_, func(error) {}, nil, CheckBranches)
-	if err != nil {
-		t.Error(err)
-	}
+	ast, _ := ParseFile(*src_, func(err error) { t.Error(err) }, nil, CheckBranches|AllowGenerics)
 
 	if ast != nil {
 		Fdump(testOut(), ast)
diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index fd97279f9d..3d1ca9d6d4 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -1924,7 +1924,7 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 	}
 
 	// distribute parameter types (len(list) > 0)
-	if named == 0 {
+	if named == 0 && !requireNames {
 		// all unnamed => found names are named types
 		for _, par := range list {
 			if typ := par.Name; typ != nil {
@@ -1932,9 +1932,6 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 				par.Name = nil
 			}
 		}
-		if requireNames {
-			p.syntaxErrorAt(list[0].Type.Pos(), "type parameters must be named")
-		}
 	} else if named != len(list) {
 		// some named => all must have names and types
 		var pos Pos // left-most error position (or unknown)
diff --git a/src/cmd/compile/internal/syntax/printer_test.go b/src/cmd/compile/internal/syntax/printer_test.go
index ec4b1de573..638e6d4a9d 100644
--- a/src/cmd/compile/internal/syntax/printer_test.go
+++ b/src/cmd/compile/internal/syntax/printer_test.go
@@ -18,11 +18,7 @@ func TestPrint(t *testing.T) {
 		t.Skip("skipping test in short mode")
 	}
 
-	// provide a no-op error handler so parsing doesn't stop after first error
-	ast, err := ParseFile(*src_, func(error) {}, nil, 0)
-	if err != nil {
-		t.Error(err)
-	}
+	ast, _ := ParseFile(*src_, func(err error) { t.Error(err) }, nil, AllowGenerics)
 
 	if ast != nil {
 		Fprint(testOut(), ast, LineForm)
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47996.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47996.go2
new file mode 100644
index 0000000000..56e90942ab
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47996.go2
@@ -0,0 +1,8 @@
+// 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 p
+
+// don't crash
+func T /* ERROR missing */ [P /* ERROR named */ ] m /* ERROR m */ () /* ERROR \) */ { /* ERROR { */ } /* ERROR } */