runtime: fix name of type parameter

CL 372774 is for reflect, this CL is for _type in runtime.
Add a test case to ensure the name method of _type can be exercised.

Updates #50208

Change-Id: I26ccf8c5c574dd9e78510cf29eb40ae7c8d449ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/393917
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/debug/heapdump_test.go

@@ -67,3 +67,29 @@ func TestWriteHeapDumpFinalizers(t *testing.T) {
 	WriteHeapDump(f.Fd())
 	println("done dump")
 }
+
+type G[T any] struct{}
+type I interface {
+	M()
+}
+
+//go:noinline
+func (g G[T]) M() {}
+
+var dummy I = G[int]{}
+var dummy2 I = G[G[int]]{}
+
+func TestWriteHeapDumpTypeName(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skipf("WriteHeapDump is not available on %s.", runtime.GOOS)
+	}
+	f, err := os.CreateTemp("", "heapdumptest")
+	if err != nil {
+		t.Fatalf("TempFile failed: %v", err)
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+	WriteHeapDump(f.Fd())
+	dummy.M()
+	dummy2.M()
+}

src/runtime/type.go

@@ -127,7 +127,14 @@ func (t *_type) name() string {
 	}
 	s := t.string()
 	i := len(s) - 1
-	for i >= 0 && s[i] != '.' {
+	sqBrackets := 0
+	for i >= 0 && (s[i] != '.' || sqBrackets != 0) {
+		switch s[i] {
+		case ']':
+			sqBrackets++
+		case '[':
+			sqBrackets--
+		}
 		i--
 	}
 	return s[i+1:]