[clang] [libcxx] [llvm] [Clang] Add warnings when mixing different charN_t types (PR #138708)

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+// RUN: %clang_cc1 -verify -fsyntax-only -std=c++20 -Wconversion %s
+
+void c8(char8_t);
+void c16(char16_t);
+void c32(char32_t);
+
+void test(char8_t u8, char16_t u16, char32_t u32) {
+    c8(u8);
+    c8(u16); // expected-warning {{implicit conversion from 'char16_t' to 'char8_t' may lose precision and change the meaning of the represented code unit}}
+    c8(u32); // expected-warning {{implicit conversion from 'char32_t' to 'char8_t' may lose precision and change the meaning of the represented code unit}}
+
+    c16(u8);  // expected-warning {{implicit conversion from 'char8_t' to 'char16_t' may change the meaning of the represented code unit}}
+    c16(u16);
+    c16(u32); // expected-warning {{implicit conversion from 'char32_t' to 'char16_t' may lose precision and change the meaning of the represented code unit}}
+
+    c32(u8);  // expected-warning {{implicit conversion from 'char8_t' to 'char32_t' may change the meaning of the represented code unit}}
+    c32(u16); // expected-warning {{implicit conversion from 'char16_t' to 'char32_t' may change the meaning of the represented code unit}}
+    c32(u32);
+
+
+    c8(char32_t(0x7f));
+    c8(char32_t(0x80));   // expected-warning {{implicit conversion from 'char32_t' to 'char8_t' changes the meaning of the codepoint '<U+0080>'}}
+
+    c8(char16_t(0x7f));
+    c8(char16_t(0x80));   // expected-warning {{implicit conversion from 'char16_t' to 'char8_t' changes the meaning of the codepoint '<U+0080>'}}
+    c8(char16_t(0xD800)); // expected-warning {{implicit conversion from 'char16_t' to 'char8_t' changes the meaning of the code unit '<0xD800>'}}
+    c8(char16_t(0xE000)); // expected-warning {{implicit conversion from 'char16_t' to 'char8_t' changes the meaning of the codepoint '<U+E000>'}}
+
+
+    c16(char32_t(0x7f));
+    c16(char32_t(0x80));
+    c16(char32_t(0xD7FF));
+    c16(char32_t(0xD800)); // expected-warning {{implicit conversion from 'char32_t' to 'char16_t' changes the meaning of the code unit '<0xD800>'}}
+    c16(char32_t(0xE000));
+    c16(char32_t(U'🐉')); // expected-warning {{implicit conversion from 'char32_t' to 'char16_t' changes the meaning of the codepoint '🐉'}}
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cor3ntin wrote:

There is a distinction between a code unit and a code point in the diagnostic message already. Which I think is useful information. But the distinction between printable and not can just be the presentation,

> We could also present both the source value and the post conversion result to make it more explicit how the meaning changes. For example:

Presenting the source is something I tried, but because one side is always going to be invalid (ie not a complete code point) it's not actually useful

> Or, if char16_t happens to be larger than 16 bits (I'm not sure we should care about this case though):

This is not a scenario in Clang


https://github.com/llvm/llvm-project/pull/138708