[stmt.ambig]

# 8 Statements [[stmt]](./#stmt)

## 8.11 Ambiguity resolution [stmt.ambig]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/statements.tex#L1367)

There is an ambiguity in the grammar involving[*expression-statement*](stmt.expr#nt:expression-statement "8.3 Expression statement [stmt.expr]")*s* and [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]")*s*: An[*expression-statement*](stmt.expr#nt:expression-statement "8.3 Expression statement [stmt.expr]") with a [function-style explicit type
conversion](expr.type.conv "7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]") as its leftmost subexpression can be
indistinguishable from a [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]") where the first[*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") starts with a ([.](#1.sentence-1)

In those cases the[*statement*](stmt.pre#nt:statement "8.1 Preamble [stmt.pre]") is considered a [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]"),
except as specified below[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/statements.tex#L1377)

[*Note [1](#note-1)*:

If the [*statement*](stmt.pre#nt:statement "8.1 Preamble [stmt.pre]") cannot syntactically be a[*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]"), there is no ambiguity,
so this rule does not apply[.](#2.sentence-1)

In some cases, the whole [*statement*](stmt.pre#nt:statement "8.1 Preamble [stmt.pre]") needs to be examined
to determine whether this is the case[.](#2.sentence-2)

This resolves the meaning
of many examples[.](#2.sentence-3)

[*Example [1](#example-1)*:

Assuming T is a[*simple-type-specifier*](dcl.type.simple#nt:simple-type-specifier "9.2.9.3 Simple type specifiers [dcl.type.simple]") ([[dcl.type.simple]](dcl.type.simple "9.2.9.3 Simple type specifiers")),

T(a)->m = 7; // expression-statement T(a)++; // expression-statement T(a,5)<<c; // expression-statement T(*d)(int); // declaration T(e)[5]; // declaration T(f) = { 1, 2 }; // declaration T(*g)(double(3)); // declaration

In the last example above, g, which is a pointer to T,
is initialized to double(3)[.](#2.sentence-5)

This is of course ill-formed for
semantic reasons, but that does not affect the syntactic analysis[.](#2.sentence-6)

— *end example*]

The remaining cases are [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]")*s*[.](#2.sentence-7)

[*Example [2](#example-2)*: class T {// ...public: T();
 T(int);
 T(int, int);};
T(a); // declaration T(*b)(); // declaration T(c)=7; // declaration T(d),e,f=3; // declarationextern int h;
T(g)(h,2); // declaration — *end example*]

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/statements.tex#L1425)

The disambiguation is purely syntactic; that is, the meaning of the
names occurring in such a statement, beyond whether they are[*type-name*](dcl.type.simple#nt:type-name "9.2.9.3 Simple type specifiers [dcl.type.simple]")*s* or not, is not generally used in or changed by the
disambiguation[.](#3.sentence-1)

Class templates are instantiated as necessary to
determine if a qualified name is a [*type-name*](dcl.type.simple#nt:type-name "9.2.9.3 Simple type specifiers [dcl.type.simple]")[.](#3.sentence-2)

Disambiguation
precedes parsing, and a statement disambiguated as a declaration may be
an ill-formed declaration[.](#3.sentence-3)

If, during parsing, lookup finds
that a name in a template argument is bound to
(part of) the declaration being parsed,
the program is ill-formed[.](#3.sentence-4)

No diagnostic is required[.](#3.sentence-5)

[*Example [3](#example-3)*: struct T1 { T1 operator()(int x) { return T1(x); }int operator=(int x) { return x; } T1(int) { }};struct T2 { T2(int) { } };int a, (*(*b)(T2))(int), c, d;

void f() {// disambiguation requires this to be parsed as a declaration: T1(a) = 3,
 T2(4), // T2 will be declared as a variable of type T1, but this will not(*(*b)(T2(c)))(int(d)); // allow the last part of the declaration to parse properly,// since it depends on T2 being a type-name} — *end example*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/statements.tex#L1458)

A syntactically ambiguous statement that can syntactically be
a [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]") with an outermost [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") with a [*trailing-return-type*](dcl.decl.general#nt:trailing-return-type "9.3.1 General [dcl.decl.general]") is considered a [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]") only if it starts with auto[.](#4.sentence-1)

[*Example [4](#example-4)*: struct M;struct S { S* operator()(); int N; int M; void mem(S s) {auto(s)()->M; // expression, S​::​M hides ​::​M}};

void f(S s) {{auto(s)()->N; // expressionauto(s)()->M; // function declaration}{ S(s)()->N; // expression S(s)()->M; // expression}} — *end example*]