9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.7 Indeterminate storage [dcl.attr.indet]

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The attribute-token indeterminate may be applied to the definition of a block variable with automatic storage duration or to a parameter-declaration of a function declaration.
No attribute-argument-clause shall be present.
The attribute specifies that the storage of an object with automatic storage duration is initially indeterminate rather than erroneous ([basic.indet]).
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If a function parameter is declared with the indeterminate attribute, it shall be so declared in the first declaration of its function.
If a function parameter is declared with the indeterminate attribute in the first declaration of its function in one translation unit and the same function is declared without the indeterminate attribute on the same parameter in its first declaration in another translation unit, the program is ill-formed, no diagnostic required.
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[Note 1: 
Reading from an uninitialized variable that is marked [[indeterminate]] can cause undefined behavior.
void f(int); void g() { int x [[indeterminate]], y; f(y); // erroneous behavior ([basic.indet]) f(x); // undefined behavior } struct T { T() {} int x; }; int h(T t [[indeterminate]]) { f(t.x); // undefined behavior when called below return 0; } int _ = h(T()); — end note]