Repro

build.sh

#!/bin/bash
ldc2 -wi -v -c test.d

test.d

pragma(cppmap, "test_c.h");

mixin(q{
    import (C++) *;
});

// Uncomment any of these to reproduce the assertion error

void fni1(StructI1 s) {}
void fni2(StructI2 s) {}
//void fni3(StructI3 s) {}
//void fni4(StructI4 s) {}
void fni5(StructI5 s) {}
void fni6(StructI6 s) {}

void fnc1(StructC1 s) {}
void fnc2(StructC2 s) {}
//void fnc3(StructC3 s) {}
//void fnc4(StructC4 s) {}
//void fnc5(StructC5 s) {}
//void fnc6(StructC6 s) {}

test_c.h

struct StructI1
{
    int a;
};

struct StructI2
{
    int a;
    int b;
};

struct StructI3
{
    int a;
    int b;
    int c;
};

struct StructI4
{
    int a;
    int b;
    int c;
    int d;
};

struct StructI5
{
    int a;
    int b;
    int c;
    int d;
    int e;
};

struct StructI6
{
    int a;
    int b;
    int c;
    int d;
    int e;
    int f;
};

struct StructC1
{
    char a;
};

struct StructC2
{
    char a;
    char b;
};

struct StructC3
{
    char a;
    char b;
    char c;
};

struct StructC4
{
    char a;
    char b;
    char c;
    char d;
};

struct StructC5
{
    char a;
    char b;
    char c;
    char d;
    char e;
};

struct StructC6
{
    char a;
    char b;
    char c;
    char d;
    char e;
    char f;
};