diff --git a/lib/std/c/builtins.zig b/lib/std/c/builtins.zig
index 2c03c1ceac..1d100369b9 100644
--- a/lib/std/c/builtins.zig
+++ b/lib/std/c/builtins.zig
@@ -188,3 +188,9 @@ pub fn __builtin_memcpy(
 pub fn __builtin_expect(expr: c_long, c: c_long) callconv(.Inline) c_long {
     return expr;
 }
+
+// __builtin_alloca_with_align is not currently implemented.
+// It is used in a run-translated-c test and a test-translate-c test to ensure that non-implemented
+// builtins are correctly demoted. If you implement __builtin_alloca_with_align, please update the
+// run-translated-c test and the test-translate-c test to use a different non-implemented builtin.
+// pub fn __builtin_alloca_with_align(size: usize, alignment: usize) callconv(.Inline) *c_void {}
diff --git a/src/translate_c.zig b/src/translate_c.zig
index d2c5cd6233..e63ced58e9 100644
--- a/src/translate_c.zig
+++ b/src/translate_c.zig
@@ -11,6 +11,7 @@ const math = std.math;
 const ast = @import("translate_c/ast.zig");
 const Node = ast.Node;
 const Tag = Node.Tag;
+const c_builtins = std.c.builtins;
 
 const CallingConvention = std.builtin.CallingConvention;
 
@@ -1526,7 +1527,7 @@ fn transImplicitCastExpr(
             return maybeSuppressResult(c, scope, result_used, ne);
         },
         .BuiltinFnToFnPtr => {
-            return transExpr(c, scope, sub_expr, result_used);
+            return transBuiltinFnExpr(c, scope, sub_expr, result_used);
         },
         .ToVoid => {
             // Should only appear in the rhs and lhs of a ConditionalOperator
@@ -1542,6 +1543,22 @@ fn transImplicitCastExpr(
     }
 }
 
+fn isBuiltinDefined(name: []const u8) bool {
+    inline for (std.meta.declarations(c_builtins)) |decl| {
+        if (std.mem.eql(u8, name, decl.name)) return true;
+    }
+    return false;
+}
+
+fn transBuiltinFnExpr(c: *Context, scope: *Scope, expr: *const clang.Expr, used: ResultUsed) TransError!Node {
+    const node = try transExpr(c, scope, expr, used);
+    if (node.castTag(.identifier)) |ident| {
+        const name = ident.data;
+        if (!isBuiltinDefined(name)) return fail(c, error.UnsupportedTranslation, expr.getBeginLoc(), "TODO implement function '{s}' in std.c.builtins", .{name});
+    }
+    return node;
+}
+
 fn transBoolExpr(
     c: *Context,
     scope: *Scope,
@@ -4759,6 +4776,10 @@ fn parseCPrimaryExprInner(c: *Context, m: *MacroCtx, scope: *Scope) ParseError!N
         },
         .Identifier => {
             const mangled_name = scope.getAlias(slice);
+            if (mem.startsWith(u8, mangled_name, "__builtin_") and !isBuiltinDefined(mangled_name)) {
+                try m.fail(c, "TODO implement function '{s}' in std.c.builtins", .{mangled_name});
+                return error.ParseError;
+            }
             return Tag.identifier.create(c.arena, builtin_typedef_map.get(mangled_name) orelse mangled_name);
         },
         .LParen => {
diff --git a/test/run_translated_c.zig b/test/run_translated_c.zig
index bda401d08c..10c3a04540 100644
--- a/test/run_translated_c.zig
+++ b/test/run_translated_c.zig
@@ -1221,4 +1221,16 @@ pub fn addCases(cases: *tests.RunTranslatedCContext) void {
         \\    return 0;
         \\}
     , "");
+
+    // See __builtin_alloca_with_align comment in std.c.builtins
+    cases.add("use of unimplemented builtin in unused function does not prevent compilation",
+        \\#include <stdlib.h>
+        \\void unused() {
+        \\    __builtin_alloca_with_align(1, 8);
+        \\}
+        \\int main(void) {
+        \\    if (__builtin_sqrt(1.0) != 1.0) abort();
+        \\    return 0;
+        \\}
+    , "");
 }
diff --git a/test/translate_c.zig b/test/translate_c.zig
index 47d7c5d9eb..6aab4736a3 100644
--- a/test/translate_c.zig
+++ b/test/translate_c.zig
@@ -3418,4 +3418,11 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
         \\pub const MAY_NEED_PROMOTION_HEX = @import("std").meta.promoteIntLiteral(c_int, 0x80000000, .hexadecimal);
         \\pub const MAY_NEED_PROMOTION_OCT = @import("std").meta.promoteIntLiteral(c_int, 0o20000000000, .octal);
     });
+
+    // See __builtin_alloca_with_align comment in std.c.builtins
+    cases.add("demote un-implemented builtins",
+        \\#define FOO(X) __builtin_alloca_with_align((X), 8)
+    , &[_][]const u8{
+        \\pub const FOO = @compileError("TODO implement function '__builtin_alloca_with_align' in std.c.builtins");
+    });
 }