Is Microsoft Visual C 's Two-Phase Template Instantiation Truly Broken, and If So, How?

Microsoft Visual C 's Misunderstood "Broken" Two-Phase Template Instantiation

Introduction

Microsoft Visual C (MSVC) has often been criticized for its alleged flawed implementation of two-phase template instantiation. This article delves into the specifics of this criticism, examining the underlying issue and providing a detailed explanation.

The Two-Phase Template Instantiation Controversy

Two-phase template instantiation is a process in C where template classes and functions are first syntactically checked for correctness, and then complete instantiation occurs at later usage sites. However, claims have been made that MSVC does not properly implement this mechanism.

Early Understanding of the Issue

Initially, it was believed that MSVC only performed basic syntax checks on template definitions, ignoring whether names used in the template were declared. However, this understanding is incomplete.

The True Nature of the Issue

The actual issue with MSVC's two-phase template instantiation comprises two interconnected layers:

Layer 1: Incorrect First Phase Lookup

MSVC fails to perform early (first phase) lookup for non-dependent expressions, such as in the example:

int foo(void*);

template<typename T> struct S {
  S() { int i = foo(0); }
};

void foo(int);

MSVC postpones this lookup to the second phase, where it mistakenly binds the expression to 'foo(int)', resulting in an error.

Layer 2: Incorrect Second Phase Lookup

The second phase of template lookup in MSVC is also flawed. While the C standard specifies that ADL-nominated namespaces are extended in the second phase, MSVC erroneously extends non-ADL lookup as well.

This can be illustrated with the following example:

namespace N {
  struct S {};
}

void bar(void *) {}

template <typename T> void foo(T *t) {
  bar(t);
}

void bar(N::S *s) {}

Despite being dependent, the call to bar(t) is incorrectly resolved to void bar(N::S *s), demonstrating the improper behavior of MSVC's second phase lookup.

Conclusion

Microsoft Visual C 's implementation of two-phase template instantiation is indeed flawed, but not in the simplistic manner originally understood. The issue is a complex interplay between incorrect first and second phase lookups that can result in errors or incorrect behavior for certain code constructions.

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