go/types, types2: report an error if constant string overflows

Set a limit of 10 GiB for strings obtained via constant string addition.

Fixes #78346.

Change-Id: I35dbdff94f3ed32bf69654f4b3da435dad9f6236
Reviewed-on: https://go-review.googlesource.com/c/go/+/761300
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>

3 files changed, 50 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types2/const.go b/src/cmd/compile/internal/types2/const.go
index 74f45a6e8b..096187f515 100644
--- a/src/cmd/compile/internal/types2/const.go
+++ b/src/cmd/compile/internal/types2/const.go
@@ -46,6 +46,14 @@ func (check *Checker) overflow(x *operand, opPos syntax.Pos) {
 		}
 		check.errorf(atPos(opPos), InvalidConstVal, "constant %soverflow", op)
 		x.val = constant.MakeUnknown()
+		return
+	}
+
+	const maxLen = 10 * 1024 * 1024
+	if x.val.Kind() == constant.String && len(constant.StringVal(x.val)) > maxLen {
+		check.error(atPos(opPos), InvalidConstVal, "constant string too long")
+		x.val = constant.MakeUnknown()
+		return
 	}
 }
 
diff --git a/src/go/types/const.go b/src/go/types/const.go
index f897df63a7..04bf47d439 100644
--- a/src/go/types/const.go
+++ b/src/go/types/const.go
@@ -48,6 +48,14 @@ func (check *Checker) overflow(x *operand, opPos token.Pos) {
 		}
 		check.errorf(atPos(opPos), InvalidConstVal, "constant %soverflow", op)
 		x.val = constant.MakeUnknown()
+		return
+	}
+
+	const maxLen = 10 * 1024 * 1024
+	if x.val.Kind() == constant.String && len(constant.StringVal(x.val)) > maxLen {
+		check.error(atPos(opPos), InvalidConstVal, "constant string too long")
+		x.val = constant.MakeUnknown()
+		return
 	}
 }
 
diff --git a/src/internal/types/testdata/fixedbugs/issue78346.go b/src/internal/types/testdata/fixedbugs/issue78346.go
new file mode 100644
index 0000000000..9db9891e13
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue78346.go
@@ -0,0 +1,34 @@
+// Copyright 2026 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
+
+const (
+	a = "x"
+	b = a + a
+	c = b + b
+	d = c + c
+	e = d + d
+	f = e + e
+	g = f + f
+	h = g + g
+	i = h + h
+	j = i + i
+	k = j + j
+	l = k + k
+	m = l + l
+	n = m + m
+	o = n + n
+	p = o + o
+	q = p + p
+	r = q + q
+	s = r + r
+	t = s + s
+	u = t + t
+	v = u + u
+	w = v + v
+	x = w + w
+	y = x + /* ERROR "constant string too long" */ x
+	z = y + y
+)