cmd/compile: fix missing source information in ssa view

Endlineno is lost when we call "genericSubst" to create the new
instantiation of the generic function. This will cause "readFuncLines"
to fail to read the target function.

To fix this issue, as @mdempsky pointed out, add the line in
cmd/compile/internal/noder/stencil.go:
    newf.Endlineno = gf.Endlineno

Fixes #51988

Change-Id: Ib408e4ed0ceb68df8dedda4fb551309e8385aada
Reviewed-on: https://go-review.googlesource.com/c/go/+/399057
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

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

@@ -741,6 +741,7 @@ func (g *genInst) genericSubst(newsym *types.Sym, nameNode *ir.Name, tparams []*
 	// Pos of the instantiated function is same as the generic function
 	newf := ir.NewFunc(gf.Pos())
 	newf.Pragma = gf.Pragma // copy over pragmas from generic function to stenciled implementation.
+	newf.Endlineno = gf.Endlineno
 	newf.Nname = ir.NewNameAt(gf.Pos(), newsym)
 	newf.Nname.Func = newf
 	newf.Nname.Defn = newf

src/runtime/callers_test.go

@@ -309,3 +309,33 @@ func TestCallersDeferNilFuncPanicWithLoop(t *testing.T) {
 	// function exit, rather than at the defer statement.
 	state = 2
 }
+
+// issue #51988
+// Func.Endlineno was lost when instantiating generic functions, leading to incorrect
+// stack trace positions.
+func TestCallersEndlineno(t *testing.T) {
+	testNormalEndlineno(t)
+	testGenericEndlineno[int](t)
+}
+
+func testNormalEndlineno(t *testing.T) {
+	defer testCallerLine(t, callerLine(t, 0)+1)
+}
+
+func testGenericEndlineno[_ any](t *testing.T) {
+	defer testCallerLine(t, callerLine(t, 0)+1)
+}
+
+func testCallerLine(t *testing.T, want int) {
+	if have := callerLine(t, 1); have != want {
+		t.Errorf("callerLine(1) returned %d, but want %d\n", have, want)
+	}
+}
+
+func callerLine(t *testing.T, skip int) int {
+	_, _, line, ok := runtime.Caller(skip + 1)
+	if !ok {
+		t.Fatalf("runtime.Caller(%d) failed", skip+1)
+	}
+	return line
+}