Improver handling of "if constexpr" non-braced substatement

[wchilders-nvidia]

Issue Summary

When a substatement of an "if constexpr" is discarded during instantiation and was not enclosed in braces, statements following the discarded statement could be additionally discarded.

Reproducing Source Code

template <bool B> constexpr int f() {
     return [] {
        if constexpr (B)
            while (false) {}
        return 1;
        return 2;
    }();    
}
static_assert(f<false>() == 1);

Command-line Options

cpfe --c++16

Type of Issue

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

Additional Details

This was observed only with lambdas because they don't use the cached positional information that normal template instantiations of "if constexpr" would.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

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