Incorrect constraint check for call to module-imported function template with deduced return type #453

^{Originally posted by cpplearner October 8, 2026}

Issue Summary

When a call to an uninstantiated function template with deduced return type appears in a requires-expression, and the function template is imported from a (MSVC-generated) IFC file, EDG evaluates the constraint to false, even though a direct call to the function template works fine.

Like #230, this bug affects the uses of IntelliSense with import std; because MSVC STL's std::ranges::begin has an operator() with a deduced return type, and std::ranges::range calls std::ranges::begin in its requires-expression.

Reproducing Source Code

mod1.ixx (compiled with MSVC's cl.exe /c /std:c++20 mod1.ixx):

export module mod1;

export template<class T>
auto fn(T&&)
{
    return 42;
}

example.cxx (compiled with \CXX\edgcpp-compiler\build\msvc-release\bin\cpfe.exe --ms_c++20 --modules example.cxx):

import mod1;

template<class T>
concept range = requires(T& t) {
    fn(t);
};

static_assert(range<int(&)[1]>); // static assertion failed, should be OK

int main() {}

Command-line Options

cpfe.exe --ms_c++20 --modules example.cxx

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

The IFC file is generated by MSVC Compiler Version 19.52.36815 for x64 (PREVIEW). The file example.cxx compiles fine with cl.exe /std:c++20 example.cxx and fails to compile with cl.exe /BE /std:c++20 example.cxx.

This is the generated IFC file (zipped):
mod1.ifc.zip

Reproducing (Standard) Configuration(s)

windows-msvc-release

Reproducing (Non-standard) Configuration

No response

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