cmd/compile: report error instead of ICE for oversized map element

When a map element or key type is 2GB or larger, the compiler crashed
with "internal compiler error: map elem too big" in reflectdata.ZeroAddr.

Add a size check in types.CalcSize for TMAP, similar to the existing
check for channel element types, so that oversized map elements are
reported as a normal error before reaching ZeroAddr.

Fixes #78355

Change-Id: I281a5f0ec7dda6dac084787859156d4409dc860c
Reviewed-on: https://go-review.googlesource.com/c/go/+/760600
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

4 files changed, 25 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types/size.go b/src/cmd/compile/internal/types/size.go
index 1acb041e35..4d64c7f8d2 100644
--- a/src/cmd/compile/internal/types/size.go
+++ b/src/cmd/compile/internal/types/size.go
@@ -366,6 +366,12 @@ func CalcSize(t *Type) {
 		t.intRegs = 1
 		CheckSize(t.Elem())
 		CheckSize(t.Key())
+		if t.Elem().width >= 1<<31 {
+			base.Errorf("map element type too large")
+		}
+		if t.Key().width >= 1<<31 {
+			base.Errorf("map key type too large")
+		}
 		t.setAlg(ANOEQ)
 		t.ptrBytes = int64(PtrSize)
 
diff --git a/src/cmd/compile/internal/types2/stdlib_test.go b/src/cmd/compile/internal/types2/stdlib_test.go
index 924b7b841f..6108155fc7 100644
--- a/src/cmd/compile/internal/types2/stdlib_test.go
+++ b/src/cmd/compile/internal/types2/stdlib_test.go
@@ -328,6 +328,7 @@ func TestStdFixed(t *testing.T) {
 		"issue48230.go",  // go/types doesn't check validity of //go:xxx directives
 		"issue49767.go",  // go/types does not have constraints on channel element size
 		"issue49814.go",  // go/types does not have constraints on array size
+		"issue78355.go",  // types2 does not have constraints on map element size
 		"issue56103.go",  // anonymous interface cycles; will be a type checker error in 1.22
 		"issue52697.go",  // types2 does not have constraints on stack size
 
diff --git a/src/go/types/stdlib_test.go b/src/go/types/stdlib_test.go
index 7830029fb2..8d4cba6625 100644
--- a/src/go/types/stdlib_test.go
+++ b/src/go/types/stdlib_test.go
@@ -330,6 +330,7 @@ func TestStdFixed(t *testing.T) {
 		"issue48230.go",  // go/types doesn't check validity of //go:xxx directives
 		"issue49767.go",  // go/types does not have constraints on channel element size
 		"issue49814.go",  // go/types does not have constraints on array size
+		"issue78355.go",  // go/types does not have constraints on map element size
 		"issue56103.go",  // anonymous interface cycles; will be a type checker error in 1.22
 		"issue52697.go",  // go/types does not have constraints on stack size
 
diff --git a/test/fixedbugs/issue78355.go b/test/fixedbugs/issue78355.go
new file mode 100644
index 0000000000..092d659f03
--- /dev/null
+++ b/test/fixedbugs/issue78355.go
@@ -0,0 +1,17 @@
+// errorcheck
+
+// 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.
+
+//go:build amd64 || arm64
+
+// Issue 78355: map element or key type too large should not cause ICE.
+
+package p
+
+type T [1 << 31]byte
+
+func F(m map[int]T) { // ERROR "map element type too large"
+	_ = m[0]
+}